bd-109: Add retry logic and race condition handling for N-way collisions

- Added ExecInTransaction helper for atomic database operations
- Added IsUniqueConstraintError to detect UNIQUE constraint violations
- Wrapped RemapCollisions with retry logic (3 attempts with counter sync)
- Enhanced handleRename to detect race conditions where target ID exists
- Added defensive checks for when old ID has been deleted by another clone

Progress: Improves N-way collision handling, though full solution requires
more work (tracked in bd-108). Tests now reach later convergence rounds
before hitting complex collision scenarios.

Amp-Thread-ID: https://ampcode.com/threads/T-2b850a80-f8bd-4e38-b661-e33d1cfa7281
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/sqlite/util.go

@@ -3,6 +3,8 @@ package sqlite
 import (
 	"context"
 	"database/sql"
+	"fmt"
+	"strings"
 )
 
 // QueryContext exposes the underlying database QueryContext method for advanced queries
@@ -16,3 +18,37 @@ func (s *SQLiteStorage) QueryContext(ctx context.Context, query string, args ...
 func (s *SQLiteStorage) BeginTx(ctx context.Context) (*sql.Tx, error) {
 	return s.db.BeginTx(ctx, nil)
 }
+
+// ExecInTransaction executes a function within a database transaction.
+// If the function returns an error, the transaction is rolled back.
+// Otherwise, the transaction is committed.
+func (s *SQLiteStorage) ExecInTransaction(ctx context.Context, fn func(*sql.Tx) error) error {
+	tx, err := s.db.BeginTx(ctx, nil)
+	if err != nil {
+		return fmt.Errorf("failed to begin transaction: %w", err)
+	}
+	defer func() { _ = tx.Rollback() }()
+
+	if err := fn(tx); err != nil {
+		return err
+	}
+
+	if err := tx.Commit(); err != nil {
+		return fmt.Errorf("failed to commit transaction: %w", err)
+	}
+
+	return nil
+}
+
+// IsUniqueConstraintError checks if an error is a UNIQUE constraint violation
+func IsUniqueConstraintError(err error) bool {
+	if err == nil {
+		return false
+	}
+	return strings.Contains(err.Error(), "UNIQUE constraint failed")
+}
+
+// isUniqueConstraintError is an alias for IsUniqueConstraintError for internal use
+func isUniqueConstraintError(err error) bool {
+	return IsUniqueConstraintError(err)
+}