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_dolt.go

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