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/configfile/configfile.go

@@ -18,6 +18,15 @@ type Config struct {
 	// Deletions configuration
 	DeletionsRetentionDays int `json:"deletions_retention_days,omitempty"` // 0 means use default (3 days)
 
+	// Dolt server mode configuration (bd-dolt.2.2)
+	// When Mode is "server", connects to external dolt sql-server instead of embedded.
+	// This enables multi-writer access for multi-agent environments.
+	DoltMode       string `json:"dolt_mode,omitempty"`        // "embedded" (default) or "server"
+	DoltServerHost string `json:"dolt_server_host,omitempty"` // Server host (default: 127.0.0.1)
+	DoltServerPort int    `json:"dolt_server_port,omitempty"` // Server port (default: 3306)
+	DoltServerUser string `json:"dolt_server_user,omitempty"` // MySQL user (default: root)
+	// Note: Password should be set via BEADS_DOLT_PASSWORD env var for security
+
 	// Deprecated: LastBdVersion is no longer used for version tracking.
 	// Version is now stored in .local_version (gitignored) to prevent
 	// upgrade notifications firing after git operations reset metadata.json.
@@ -185,3 +194,53 @@ func (c *Config) GetBackend() string {
 	}
 	return c.Backend
 }
+
+// Dolt mode constants
+const (
+	DoltModeEmbedded = "embedded"
+	DoltModeServer   = "server"
+)
+
+// Default Dolt server settings
+const (
+	DefaultDoltServerHost = "127.0.0.1"
+	DefaultDoltServerPort = 3306
+	DefaultDoltServerUser = "root"
+)
+
+// IsDoltServerMode returns true if Dolt is configured for server mode.
+func (c *Config) IsDoltServerMode() bool {
+	return c.GetBackend() == BackendDolt && c.DoltMode == DoltModeServer
+}
+
+// GetDoltMode returns the Dolt connection mode, defaulting to embedded.
+func (c *Config) GetDoltMode() string {
+	if c.DoltMode == "" {
+		return DoltModeEmbedded
+	}
+	return c.DoltMode
+}
+
+// GetDoltServerHost returns the Dolt server host, defaulting to 127.0.0.1.
+func (c *Config) GetDoltServerHost() string {
+	if c.DoltServerHost == "" {
+		return DefaultDoltServerHost
+	}
+	return c.DoltServerHost
+}
+
+// GetDoltServerPort returns the Dolt server port, defaulting to 3306.
+func (c *Config) GetDoltServerPort() int {
+	if c.DoltServerPort == 0 {
+		return DefaultDoltServerPort
+	}
+	return c.DoltServerPort
+}
+
+// GetDoltServerUser returns the Dolt server user, defaulting to root.
+func (c *Config) GetDoltServerUser() string {
+	if c.DoltServerUser == "" {
+		return DefaultDoltServerUser
+	}
+	return c.DoltServerUser
+}