Add Beads library API for Go integration

Expose full Storage interface and all types through public beads.go API,
enabling external Go projects (like VC) to import Beads directly instead
of spawning CLI processes.

Changes:
- Expanded beads.go with all public types (Issue, Dependency, Comment, etc.)
- Added all constants (Status, IssueType, DependencyType, EventType)
- Created comprehensive integration tests (beads_integration_test.go)
- Added library usage example at examples/library-usage/
- Documented library integration in README.md

Test coverage: 96.4% on public API, 14 integration tests, all passing.

Closes bd-58, bd-59

Amp-Thread-ID: https://ampcode.com/threads/T-f0093c79-7422-45e2-b0ed-0ddfebc9ffea
Co-authored-by: Amp <amp@ampcode.com>

examples/library-usage/main.go

@@ -0,0 +1,129 @@
+// Package main demonstrates using Beads as a Go library
+//
+// This example shows how an external project (like VC) can import and use
+// Beads programmatically instead of spawning CLI processes.
+package main
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"time"
+
+	"github.com/steveyegge/beads"
+)
+
+func main() {
+	ctx := context.Background()
+
+	// Find the Beads database (looks for .beads/*.db in current/parent dirs)
+	dbPath := beads.FindDatabasePath()
+	if dbPath == "" {
+		log.Fatal("No Beads database found. Run 'bd init' first.")
+	}
+
+	fmt.Printf("Using database: %s\n\n", dbPath)
+
+	// Open the database
+	store, err := beads.NewSQLiteStorage(dbPath)
+	if err != nil {
+		log.Fatalf("Failed to open storage: %v", err)
+	}
+	defer store.Close()
+
+	// Example 1: Get ready work
+	fmt.Println("=== Ready Work ===")
+	ready, err := store.GetReadyWork(ctx, beads.WorkFilter{
+		Status: beads.StatusOpen,
+		Limit:  5,
+	})
+	if err != nil {
+		log.Fatalf("Failed to get ready work: %v", err)
+	}
+
+	for _, issue := range ready {
+		fmt.Printf("- %s: %s (priority %d)\n", issue.ID, issue.Title, issue.Priority)
+	}
+
+	// Example 2: Create an issue
+	fmt.Println("\n=== Creating Issue ===")
+	newIssue := &beads.Issue{
+		ID:          "", // Empty = auto-generate
+		Title:       "Example library-created issue",
+		Description: "This issue was created programmatically using Beads as a library",
+		Status:      beads.StatusOpen,
+		Priority:    2,
+		IssueType:   beads.TypeTask,
+		CreatedAt:   time.Now(),
+		UpdatedAt:   time.Now(),
+	}
+
+	if err := store.CreateIssue(ctx, newIssue, "library-example"); err != nil {
+		log.Fatalf("Failed to create issue: %v", err)
+	}
+
+	fmt.Printf("Created issue: %s\n", newIssue.ID)
+
+	// Example 3: Add a dependency
+	fmt.Println("\n=== Adding Dependency ===")
+	dep := &beads.Dependency{
+		IssueID:     newIssue.ID,
+		DependsOnID: "bd-1", // Assumes bd-1 exists
+		Type:        beads.DepDiscoveredFrom,
+		CreatedAt:   time.Now(),
+		CreatedBy:   "library-example",
+	}
+
+	if err := store.AddDependency(ctx, dep, "library-example"); err != nil {
+		// Don't fail if bd-1 doesn't exist
+		fmt.Printf("Note: Could not add dependency (bd-1 may not exist): %v\n", err)
+	} else {
+		fmt.Printf("Added dependency: %s discovered-from bd-1\n", newIssue.ID)
+	}
+
+	// Example 4: Add a label
+	fmt.Println("\n=== Adding Label ===")
+	if err := store.AddLabel(ctx, newIssue.ID, "library-usage", "library-example"); err != nil {
+		log.Fatalf("Failed to add label: %v", err)
+	}
+	fmt.Printf("Added label 'library-usage' to %s\n", newIssue.ID)
+
+	// Example 5: Add a comment
+	fmt.Println("\n=== Adding Comment ===")
+	comment, err := store.AddIssueComment(ctx, newIssue.ID, "library-example", "This is a programmatic comment")
+	if err != nil {
+		log.Fatalf("Failed to add comment: %v", err)
+	}
+	fmt.Printf("Added comment #%d\n", comment.ID)
+
+	// Example 6: Update issue status
+	fmt.Println("\n=== Updating Status ===")
+	updates := map[string]interface{}{
+		"status": beads.StatusInProgress,
+	}
+	if err := store.UpdateIssue(ctx, newIssue.ID, updates, "library-example"); err != nil {
+		log.Fatalf("Failed to update issue: %v", err)
+	}
+	fmt.Printf("Updated %s status to in_progress\n", newIssue.ID)
+
+	// Example 7: Get statistics
+	fmt.Println("\n=== Statistics ===")
+	stats, err := store.GetStatistics(ctx)
+	if err != nil {
+		log.Fatalf("Failed to get statistics: %v", err)
+	}
+
+	fmt.Printf("Total issues: %d\n", stats.TotalIssues)
+	fmt.Printf("Open: %d | In Progress: %d | Closed: %d | Blocked: %d | Ready: %d\n",
+		stats.OpenIssues, stats.InProgressIssues, stats.ClosedIssues,
+		stats.BlockedIssues, stats.ReadyIssues)
+
+	// Example 8: Close the issue
+	fmt.Println("\n=== Closing Issue ===")
+	if err := store.CloseIssue(ctx, newIssue.ID, "Completed demo", "library-example"); err != nil {
+		log.Fatalf("Failed to close issue: %v", err)
+	}
+	fmt.Printf("Closed issue %s\n", newIssue.ID)
+
+	fmt.Println("\n✅ Library usage demo complete!")
+}