feat(storage): add --backend flag for Dolt backend selection

Phase 2 of Dolt integration - enables runtime backend selection:

- Add --backend flag to bd init (sqlite|dolt)
- Create storage factory for backend instantiation
- Update daemon and main.go to use factory with config detection
- Update database discovery to find Dolt backends via metadata.json
- Fix Dolt schema init to split statements for MySQL compatibility
- Add ReadOnly mode to skip schema init for read-only commands

Usage: bd init --backend dolt --prefix myproject

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

internal/storage/factory/factory.go

@@ -0,0 +1,86 @@
+// Package factory provides functions for creating storage backends based on configuration.
+package factory
+
+import (
+	"context"
+	"fmt"
+	"path/filepath"
+	"time"
+
+	"github.com/steveyegge/beads/internal/configfile"
+	"github.com/steveyegge/beads/internal/storage"
+	"github.com/steveyegge/beads/internal/storage/dolt"
+	"github.com/steveyegge/beads/internal/storage/sqlite"
+)
+
+// Options configures how the storage backend is opened
+type Options struct {
+	ReadOnly    bool
+	LockTimeout time.Duration
+}
+
+// New creates a storage backend based on the backend type.
+// For SQLite, path should be the full path to the .db file.
+// For Dolt, path should be the directory containing the Dolt database.
+func New(ctx context.Context, backend, path string) (storage.Storage, error) {
+	return NewWithOptions(ctx, backend, path, Options{})
+}
+
+// NewWithOptions creates a storage backend with the specified options.
+func NewWithOptions(ctx context.Context, backend, path string, opts Options) (storage.Storage, error) {
+	switch backend {
+	case configfile.BackendSQLite, "":
+		if opts.ReadOnly {
+			if opts.LockTimeout > 0 {
+				return sqlite.NewReadOnlyWithTimeout(ctx, path, opts.LockTimeout)
+			}
+			return sqlite.NewReadOnly(ctx, path)
+		}
+		if opts.LockTimeout > 0 {
+			return sqlite.NewWithTimeout(ctx, path, opts.LockTimeout)
+		}
+		return sqlite.New(ctx, path)
+	case configfile.BackendDolt:
+		return dolt.New(ctx, &dolt.Config{Path: path, ReadOnly: opts.ReadOnly})
+	default:
+		return nil, fmt.Errorf("unknown storage backend: %s (supported: sqlite, dolt)", backend)
+	}
+}
+
+// NewFromConfig creates a storage backend based on the metadata.json configuration.
+// beadsDir is the path to the .beads directory.
+func NewFromConfig(ctx context.Context, beadsDir string) (storage.Storage, error) {
+	return NewFromConfigWithOptions(ctx, beadsDir, Options{})
+}
+
+// NewFromConfigWithOptions creates a storage backend with options from metadata.json.
+func NewFromConfigWithOptions(ctx context.Context, beadsDir string, opts Options) (storage.Storage, error) {
+	cfg, err := configfile.Load(beadsDir)
+	if err != nil {
+		return nil, fmt.Errorf("loading config: %w", err)
+	}
+	if cfg == nil {
+		cfg = configfile.DefaultConfig()
+	}
+
+	backend := cfg.GetBackend()
+	switch backend {
+	case configfile.BackendSQLite:
+		return NewWithOptions(ctx, backend, cfg.DatabasePath(beadsDir), opts)
+	case configfile.BackendDolt:
+		// For Dolt, use a subdirectory to store the Dolt database
+		doltPath := filepath.Join(beadsDir, "dolt")
+		return NewWithOptions(ctx, backend, doltPath, opts)
+	default:
+		return nil, fmt.Errorf("unknown storage backend in config: %s", backend)
+	}
+}
+
+// GetBackendFromConfig returns the backend type from metadata.json
+func GetBackendFromConfig(beadsDir string) string {
+	cfg, err := configfile.Load(beadsDir)
+	if err != nil || cfg == nil {
+		return configfile.BackendSQLite
+	}
+	return cfg.GetBackend()
+}