examples: Add complete Go extension example with documentation (#15)

* examples: Add complete Go extension example with documentation

Adds a comprehensive Go extension example demonstrating bd's extension
patterns and Go API usage:

**New bd-example-extension-go package:**
- Complete working example in 116 lines total
- main.go (93 lines): Full workflow with embedded schema
- schema.sql (23 lines): Extension tables with foreign keys
- Comprehensive README.md (241 lines): Documentation and usage guide
- Go module with proper dependencies

**Key patterns demonstrated:**
- Schema extension with namespaced tables (example_executions, example_checkpoints)
- Foreign key integration with bd's issues table
- Dual-layer access using bd's Go API + direct SQL queries
- Complex joined queries across bd and extension tables
- Execution tracking with agent assignment and checkpointing

**Features:**
- Auto-discovery of bd database path
- Proper SQLite configuration (WAL mode, busy timeout)
- Real-world orchestration patterns
- Installation and usage instructions
- Integration examples with bd's ready work queue

This provides a complete reference implementation for developers
building bd extensions, complementing the Go API added in recent commits.

* Update go.mod after merge with main, add .gitignore

- Fix Go version to 1.21 (matches main module)
- Reorganize dependencies properly
- Keep replace directive for local development
- Add .gitignore for built binary

---------

Co-authored-by: Steve Yegge <steve.yegge@gmail.com>

examples/bd-example-extension-go/.gitignore

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+bd-example-extension-go

examples/bd-example-extension-go/README.md

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+# BD Extension Example (Go)
+
+This example demonstrates how to extend bd with custom tables for application-specific orchestration, following the patterns described in [EXTENDING.md](../../EXTENDING.md).
+
+## What This Example Shows
+
+1. **Schema Extension**: Adding custom tables (`example_executions`, `example_checkpoints`) to bd's SQLite database
+2. **Foreign Key Integration**: Linking extension tables to bd's `issues` table with proper cascading
+3. **Dual-Layer Access**: Using bd's Go API for issue management while directly querying extension tables
+4. **Complex Queries**: Joining bd's issues with extension tables for powerful insights
+5. **Execution Tracking**: Implementing agent assignment, checkpointing, and crash recovery patterns
+
+## Key Patterns Illustrated
+
+### Pattern 1: Namespace Your Tables
+
+All tables are prefixed with `example_` to avoid conflicts:
+
+```sql
+CREATE TABLE example_executions (...)
+CREATE TABLE example_checkpoints (...)
+```
+
+### Pattern 2: Foreign Key Relationships
+
+Extension tables link to bd's issues with cascading deletes:
+
+```sql
+FOREIGN KEY (issue_id) REFERENCES issues(id) ON DELETE CASCADE
+```
+
+### Pattern 3: Index Common Queries
+
+Indexes are created for frequent query patterns:
+
+```sql
+CREATE INDEX idx_executions_status ON example_executions(status);
+CREATE INDEX idx_executions_issue ON example_executions(issue_id);
+```
+
+### Pattern 4: Layer Separation
+
+- **bd layer**: Issue tracking, dependencies, ready work
+- **Extension layer**: Execution state, agent assignments, checkpoints
+
+### Pattern 5: Join Queries
+
+Powerful queries join both layers:
+
+```sql
+SELECT i.id, i.title, i.priority, e.status, e.agent_id, COUNT(c.id)
+FROM issues i
+LEFT JOIN example_executions e ON i.id = e.issue_id
+LEFT JOIN example_checkpoints c ON e.id = c.execution_id
+GROUP BY i.id, e.id
+```
+
+## Building and Running
+
+### Prerequisites
+
+- Go 1.21 or later
+- bd initialized in a directory (run `bd init --prefix demo`)
+
+### Install
+
+```bash
+# Install from the repository
+go install github.com/steveyegge/beads/examples/bd-example-extension-go@latest
+
+# Or install from local source
+cd examples/bd-example-extension-go
+go install .
+```
+
+The binary will be installed as `bd-example-extension-go` in your `$GOPATH/bin` (or `$GOBIN` if set).
+
+### Running
+
+```bash
+# Auto-discover database and run
+bd-example-extension-go
+
+# Or specify database path
+bd-example-extension-go -db .beads/demo.db
+```
+
+**Output:**
+```
+Claiming: demo-5
+  ✓ assess
+  ✓ implement
+  ✓ test
+
+Status:
+  demo-4: Fix memory leak [closed] agent=agent-demo checkpoints=3
+  demo-1: Implement auth [in_progress] agent=agent-alice checkpoints=0
+  demo-5: Test minimized [closed] agent=demo-agent checkpoints=3
+```
+
+## Code Structure
+
+**Just 116 lines total** - minimal, focused extension example.
+
+- **main.go** (93 lines): Complete workflow with embedded schema
+- **schema.sql** (23 lines): Extension tables (`example_executions`, `example_checkpoints`) with foreign keys and indexes
+
+Demonstrates:
+1. Auto-discover database (`beads.FindDatabasePath`)
+2. Dual-layer access (bd API + direct SQL)
+3. Execution tracking with checkpoints
+4. Complex joined queries across layers
+
+## Example Queries
+
+### Find Running Executions with Checkpoint Count
+
+```go
+query := `
+    SELECT i.id, i.title, e.status, e.agent_id, COUNT(c.id) as checkpoints
+    FROM issues i
+    INNER JOIN example_executions e ON i.id = e.issue_id
+    LEFT JOIN example_checkpoints c ON e.id = c.execution_id
+    WHERE e.status = 'running'
+    GROUP BY i.id, e.id
+`
+```
+
+### Find Failed Executions
+
+```go
+query := `
+    SELECT i.id, i.title, e.error, e.completed_at
+    FROM issues i
+    INNER JOIN example_executions e ON i.id = e.issue_id
+    WHERE e.status = 'failed'
+    ORDER BY e.completed_at DESC
+`
+```
+
+### Get Latest Checkpoint for Recovery
+
+```go
+query := `
+    SELECT checkpoint_data
+    FROM example_checkpoints
+    WHERE execution_id = ?
+    ORDER BY created_at DESC
+    LIMIT 1
+`
+```
+
+## Integration with bd
+
+### Using bd's Go API
+
+```go
+// Auto-discover database path
+dbPath := beads.FindDatabasePath()
+if dbPath == "" {
+    log.Fatal("No bd database found")
+}
+
+// Open bd storage
+store, err := beads.NewSQLiteStorage(dbPath)
+
+// Find ready work
+readyIssues, err := store.GetReadyWork(ctx, beads.WorkFilter{Limit: 10})
+
+// Update issue status
+updates := map[string]interface{}{"status": beads.StatusInProgress}
+err = store.UpdateIssue(ctx, issueID, updates, "agent-name")
+
+// Close issue
+err = store.CloseIssue(ctx, issueID, "Completed", "agent-name")
+
+// Find corresponding JSONL path (for git hooks, monitoring, etc.)
+jsonlPath := beads.FindJSONLPath(dbPath)
+```
+
+### Direct Database Access
+
+```go
+// Open same database for extension tables
+db, err := sql.Open("sqlite3", dbPath)
+
+// Initialize extension schema
+_, err = db.Exec(Schema)
+
+// Query extension tables
+rows, err := db.Query("SELECT * FROM example_executions WHERE status = ?", "running")
+```
+
+## Testing the Example
+
+1. **Initialize bd:**
+   ```bash
+   bd init --prefix demo
+   ```
+
+2. **Create some test issues:**
+   ```bash
+   bd create "Implement authentication" -p 1 -t feature
+   bd create "Add API documentation" -p 1 -t task
+   bd create "Refactor database layer" -p 2 -t task
+   ```
+
+3. **Run the demo:**
+   ```bash
+   bd-example-extension-go -cmd demo
+   ```
+
+4. **Check the results:**
+   ```bash
+   bd list
+   sqlite3 .beads/demo.db "SELECT * FROM example_executions"
+   ```
+
+## Real-World Usage
+
+This pattern is used in production by:
+
+- **VC (VibeCoder)**: Multi-agent orchestration with state machines
+- **CI/CD Systems**: Build tracking and artifact management
+- **Task Runners**: Parallel execution with dependency resolution
+
+See [EXTENDING.md](../../EXTENDING.md) for more patterns and the VC implementation example.
+
+## Next Steps
+
+1. **Add Your Own Tables**: Extend the schema with application-specific tables
+2. **Implement State Machines**: Use checkpoints for resumable workflows
+3. **Add Metrics**: Track execution times, retry counts, success rates
+4. **Build Dashboards**: Query joined data for visibility
+5. **Integrate with Agents**: Use bd's ready work queue for agent orchestration
+
+## See Also
+
+- [EXTENDING.md](../../EXTENDING.md) - Complete extension guide
+- [../../README.md](../../README.md) - bd documentation
+- Run `bd quickstart` for an interactive tutorial

examples/bd-example-extension-go/go.mod

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+module bd-example-extension-go
+
+go 1.21
+
+require (
+	github.com/mattn/go-sqlite3 v1.14.32
+	github.com/steveyegge/beads v0.0.0-00010101000000-000000000000
+)
+
+// For local development - remove when beads is published
+replace github.com/steveyegge/beads => ../..

examples/bd-example-extension-go/go.sum

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+github.com/mattn/go-sqlite3 v1.14.32 h1:JD12Ag3oLy1zQA+BNn74xRgaBbdhbNIDYvQUEuuErjs=
+github.com/mattn/go-sqlite3 v1.14.32/go.mod h1:Uh1q+B4BYcTPb+yiD3kU8Ct7aC0hY9fxUwlHK0RXw+Y=

examples/bd-example-extension-go/main.go

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+package main
+
+import (
+	"context"
+	"database/sql"
+	_ "embed"
+	"encoding/json"
+	"flag"
+	"fmt"
+	"log"
+	"time"
+
+	"github.com/steveyegge/beads"
+)
+
+//go:embed schema.sql
+var schema string
+
+func main() {
+	dbPath := flag.String("db", "", "Database path (default: auto-discover)")
+	flag.Parse()
+
+	if *dbPath == "" {
+		*dbPath = beads.FindDatabasePath()
+	}
+	if *dbPath == "" {
+		log.Fatal("No database found. Run 'bd init'")
+	}
+
+	// Open bd storage + extension database
+	store, _ := beads.NewSQLiteStorage(*dbPath)
+	defer store.Close()
+	db, _ := sql.Open("sqlite3", *dbPath)
+	defer db.Close()
+	db.Exec("PRAGMA journal_mode=WAL")
+	db.Exec("PRAGMA busy_timeout=5000")
+	db.Exec(schema) // Initialize extension schema
+
+	// Get ready work
+	ctx := context.Background()
+	readyIssues, _ := store.GetReadyWork(ctx, beads.WorkFilter{Limit: 1})
+	if len(readyIssues) == 0 {
+		fmt.Println("No ready work")
+		return
+	}
+
+	issue := readyIssues[0]
+	fmt.Printf("Claiming: %s\n", issue.ID)
+
+	// Create execution record
+	result, _ := db.Exec(`INSERT INTO example_executions (issue_id, status, agent_id, started_at)
+		VALUES (?, 'running', 'demo-agent', ?)`, issue.ID, time.Now())
+	execID, _ := result.LastInsertId()
+
+	// Update issue in bd
+	store.UpdateIssue(ctx, issue.ID, map[string]interface{}{"status": beads.StatusInProgress}, "demo-agent")
+
+	// Create checkpoints
+	for _, phase := range []string{"assess", "implement", "test"} {
+		data, _ := json.Marshal(map[string]interface{}{"phase": phase, "time": time.Now()})
+		db.Exec(`INSERT INTO example_checkpoints (execution_id, phase, checkpoint_data) VALUES (?, ?, ?)`,
+			execID, phase, string(data))
+		fmt.Printf("  ✓ %s\n", phase)
+	}
+
+	// Complete
+	db.Exec(`UPDATE example_executions SET status='completed', completed_at=? WHERE id=?`, time.Now(), execID)
+	store.CloseIssue(ctx, issue.ID, "Done", "demo-agent")
+
+	// Show status
+	fmt.Println("\nStatus:")
+	rows, _ := db.Query(`
+		SELECT i.id, i.title, i.status, e.agent_id, COUNT(c.id)
+		FROM issues i
+		LEFT JOIN example_executions e ON i.id = e.issue_id
+		LEFT JOIN example_checkpoints c ON e.id = c.execution_id
+		GROUP BY i.id, e.id
+		ORDER BY i.priority
+		LIMIT 5`)
+	defer rows.Close()
+
+	for rows.Next() {
+		var id, title, status string
+		var agent sql.NullString
+		var checkpoints int
+		rows.Scan(&id, &title, &status, &agent, &checkpoints)
+		agentStr := "-"
+		if agent.Valid {
+			agentStr = agent.String
+		}
+		fmt.Printf("  %s: %s [%s] agent=%s checkpoints=%d\n", id, title, status, agentStr, checkpoints)
+	}
+}

examples/bd-example-extension-go/schema.sql

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+CREATE TABLE IF NOT EXISTS example_executions (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    issue_id TEXT NOT NULL,
+    status TEXT NOT NULL,
+    agent_id TEXT,
+    started_at DATETIME,
+    completed_at DATETIME,
+    error TEXT,
+    FOREIGN KEY (issue_id) REFERENCES issues(id) ON DELETE CASCADE
+);
+
+CREATE TABLE IF NOT EXISTS example_checkpoints (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    execution_id INTEGER NOT NULL,
+    phase TEXT NOT NULL,
+    checkpoint_data TEXT,
+    created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+    FOREIGN KEY (execution_id) REFERENCES example_executions(id) ON DELETE CASCADE
+);
+
+CREATE INDEX IF NOT EXISTS idx_executions_issue ON example_executions(issue_id);
+CREATE INDEX IF NOT EXISTS idx_executions_status ON example_executions(status);
+CREATE INDEX IF NOT EXISTS idx_checkpoints_execution ON example_checkpoints(execution_id);