feat: add gate issue type and CLI commands for async coordination (bd-udsi)

Add async gates - coordination primitives for agents to wait on external events
like CI completion, PR merges, timers, or human approval.

Changes:
- Add 'gate' issue type to types.go with gate-specific fields:
  - AwaitType: condition type (gh:run, gh:pr, timer, human, mail)
  - AwaitID: condition identifier
  - Timeout: max wait duration
  - Waiters: mail addresses to notify when gate clears
- Add SQLite migration 027_gate_columns for new fields
- Update all SQLite storage queries to handle gate fields
- Add bd gate commands: create, show, list, close, wait
- All commands support --json output and --no-daemon mode

Closes: bd-2v0f, bd-lz49, bd-u66e

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/sqlite/migrations/027_gate_columns.go

@@ -0,0 +1,51 @@
+package migrations
+
+import (
+	"database/sql"
+	"fmt"
+)
+
+// MigrateGateColumns adds gate-related columns to the issues table for async coordination (bd-udsi).
+// Gate fields enable agents to wait on external conditions (CI completion, human approval, etc.)
+func MigrateGateColumns(db *sql.DB) error {
+	columns := []struct {
+		name    string
+		sqlType string
+	}{
+		{"await_type", "TEXT"},          // Condition type: gh:run, gh:pr, timer, human, mail
+		{"await_id", "TEXT"},            // Condition identifier (e.g., run ID, PR number)
+		{"timeout_ns", "INTEGER"},       // Timeout in nanoseconds (Go's time.Duration)
+		{"waiters", "TEXT"},             // JSON array of mail addresses to notify
+	}
+
+	for _, col := range columns {
+		// Check if column already exists
+		var columnExists bool
+		err := db.QueryRow(`
+			SELECT COUNT(*) > 0
+			FROM pragma_table_info('issues')
+			WHERE name = ?
+		`, col.name).Scan(&columnExists)
+		if err != nil {
+			return fmt.Errorf("failed to check %s column: %w", col.name, err)
+		}
+
+		if columnExists {
+			continue
+		}
+
+		// Add the column
+		_, err = db.Exec(fmt.Sprintf(`ALTER TABLE issues ADD COLUMN %s %s`, col.name, col.sqlType))
+		if err != nil {
+			return fmt.Errorf("failed to add %s column: %w", col.name, err)
+		}
+	}
+
+	// Add index for gate type issues (for efficient gate queries)
+	_, err := db.Exec(`CREATE INDEX IF NOT EXISTS idx_issues_gate ON issues(issue_type) WHERE issue_type = 'gate'`)
+	if err != nil {
+		return fmt.Errorf("failed to create gate index: %w", err)
+	}
+
+	return nil
+}