feat: add MCP integration for external tool servers

Implement Model Context Protocol (MCP) support so Flynn can spawn MCP
server processes, discover their tools, and make them available to the
agent alongside builtin tools.

- McpClient: wraps @modelcontextprotocol/sdk with StdioClientTransport
  for process lifecycle, tool discovery (listTools), and invocation (callTool)
- McpManager: lifecycle management for multiple MCP servers with
  startAll/stopAll/restart, tool bridging into ToolRegistry
- Bridge: converts MCP tools to Flynn Tool interface with mcp:<server>:<tool>
  namespacing to avoid collisions with builtin tools
- Config: add env and cwd fields to mcp server schema
- ToolRegistry: add unregister() method for MCP server cleanup
- Daemon: wire McpManager into startup and shutdown lifecycle
- Tests: 28 new tests (bridge, manager, registry unregister)

src/mcp/types.ts

@@ -0,0 +1,49 @@
+/**
+ * MCP (Model Context Protocol) type definitions.
+ *
+ * Internal types for Flynn's MCP client integration.
+ * These wrap the SDK types into simpler forms used by the bridge and manager.
+ */
+
+/** Configuration for a single MCP server from flynn.yaml. */
+export interface McpServerConfig {
+  /** Unique name for this server (used as tool name prefix). */
+  name: string;
+  /** Executable command to spawn the server process. */
+  command: string;
+  /** Arguments passed to the command. */
+  args: string[];
+  /** Optional environment variables for the server process. */
+  env?: Record<string, string>;
+  /** Optional working directory for the server process. */
+  cwd?: string;
+}
+
+/** Connection status of an MCP server. */
+export type McpServerStatus = 'disconnected' | 'connecting' | 'connected' | 'error';
+
+/** Information about a tool discovered from an MCP server. */
+export interface McpToolInfo {
+  /** Tool name as reported by the MCP server. */
+  name: string;
+  /** Human-readable description. */
+  description: string;
+  /** JSON Schema for the tool's input parameters. */
+  inputSchema: {
+    type: 'object';
+    properties?: Record<string, unknown>;
+    required?: string[];
+  };
+}
+
+/** Runtime state of a connected MCP server. */
+export interface McpServerState {
+  /** Config this server was started from. */
+  config: McpServerConfig;
+  /** Current connection status. */
+  status: McpServerStatus;
+  /** Tools discovered from this server. */
+  tools: McpToolInfo[];
+  /** Error message if status is 'error'. */
+  error?: string;
+}