feat(dolt): add server mode config to metadata.json schema (bd-dolt.2.2)

Add Dolt server mode configuration to metadata.json for multi-writer access:

- Add DoltMode, DoltServerHost, DoltServerPort, DoltServerUser fields to Config
- Add helper methods with sensible defaults (127.0.0.1:3306, root user)
- Update factory to read server mode config and pass to dolt.Config
- Add --server, --server-host, --server-port, --server-user flags to bd init
- Validate that --server requires --backend dolt
- Add comprehensive tests for server mode configuration

Example metadata.json for server mode:
{
  "backend": "dolt",
  "database": "dolt",
  "dolt_mode": "server",
  "dolt_server_host": "192.168.1.100",
  "dolt_server_port": 3306,
  "dolt_server_user": "beads"
}

Password should be set via BEADS_DOLT_PASSWORD env var for security.

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

internal/storage/factory/factory.go

@@ -31,6 +31,7 @@ type Options struct {
 	ServerMode bool   // Connect to dolt sql-server instead of embedded
 	ServerHost string // Server host (default: 127.0.0.1)
 	ServerPort int    // Server port (default: 3306)
+	ServerUser string // MySQL user (default: root)
 }
 
 // New creates a storage backend based on the backend type.
@@ -88,6 +89,19 @@ func NewFromConfigWithOptions(ctx context.Context, beadsDir string, opts Options
 	case configfile.BackendSQLite:
 		return NewWithOptions(ctx, backend, cfg.DatabasePath(beadsDir), opts)
 	case configfile.BackendDolt:
+		// Merge Dolt server mode config into options (config provides defaults, opts can override)
+		if cfg.IsDoltServerMode() {
+			opts.ServerMode = true
+			if opts.ServerHost == "" {
+				opts.ServerHost = cfg.GetDoltServerHost()
+			}
+			if opts.ServerPort == 0 {
+				opts.ServerPort = cfg.GetDoltServerPort()
+			}
+			if opts.ServerUser == "" {
+				opts.ServerUser = cfg.GetDoltServerUser()
+			}
+		}
 		return NewWithOptions(ctx, backend, cfg.DatabasePath(beadsDir), opts)
 	default:
 		return nil, fmt.Errorf("unknown storage backend in config: %s", backend)

internal/storage/factory/factory_dolt.go

@@ -47,6 +47,7 @@ func init() {
 			ServerMode: opts.ServerMode,
 			ServerHost: opts.ServerHost,
 			ServerPort: opts.ServerPort,
+			ServerUser: opts.ServerUser,
 		})
 	})
 }