expose the clide command surface over MCP (T-225)

External MCP clients (Cursor, Windsurf, Copilot) can now drive clide. The
MCP server's tools/list is generated from the co-registered command+schema
registry (D-74) that already feeds the CLI and palette — the full
mcp__clide__* namespace with no hand-maintained second surface (D-86). Each
command's CommandSchema maps to a JSON-Schema inputSchema; tools/call routes
mcp__clide__<cmd> to dispatcher.dispatch and renders the IpcResponse as MCP
content (data as JSON, errors with isError).

register() gains a mcpExpose flag (default true); pane.tail opts out as a
poor request/response fit. tail/events are server-intercepted so they're
naturally absent. The two /ide stubs (getDiagnostics, executeCode) are left
as stubs — making them real (analyzer hook, Jupyter eval) is out of scope
per the ticket. Transport unchanged (SSE, D-73).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/main.dart

@@ -151,7 +151,7 @@ Future<void> main() async {
       ipcLog.error('ipc', 'server start failed', error: e, stackTrace: st);
       return;
     }
-    final mcp = McpServer(workspaceRoot: workRoot.path, log: ipcLog);
+    final mcp = McpServer(workspaceRoot: workRoot.path, log: ipcLog, dispatcher: dispatcher);
     mcpServer = mcp;
     try {
       await mcp.start();