feat: add MCP server for AI agent integration

Add HTTP-based MCP server on port 34438 that exposes Effect DevTools
data to AI agents via Model Context Protocol.

Tools available:
- list_spans: Query spans with filtering by status, traceId, name
- get_span: Get detailed span info including events
- get_active_spans: Get currently running spans
- list_clients: List connected Effect applications
- get_metrics: Query metrics with filtering
- get_span_tree: Get hierarchical span tree for a trace

Implementation uses @effect/ai McpServer with BunHttpServer,
running separately from the WebSocket DevTools server (port 34437).

Author: DanielFGray

bun.lock

@@ -36,11 +36,13 @@
     "client": "bun example/client.ts"
   },
   "dependencies": {
+    "@effect/ai": "^0.32.1",
     "@effect/experimental": "^0.57.11",
     "@effect/language-service": "^0.62.1",
     "@effect/platform": "^0.93.6",
     "@effect/platform-bun": "^0.86.0",
     "@effect/platform-node": "^0.103.0",
+    "@effect/rpc": "^0.72.2",
     "@opentui/core": "^0.1.60",
     "@opentui/solid": "^0.1.60",
     "dagre": "^0.8.5",

package.json

@@ -0,0 +1,75 @@
+/**
+ * MCP Server Integration
+ *
+ * This module creates the MCP server layer using @effect/ai's McpServer.
+ * It registers the DevTools toolkit and provides the StoreReader dependency.
+ */
+
+import * as McpServer from "@effect/ai/McpServer";
+import { HttpRouter, HttpServer, HttpMiddleware } from "@effect/platform";
+import { BunHttpServer } from "@effect/platform-bun";
+import * as Layer from "effect/Layer";
+import * as Effect from "effect/Effect";
+import { DevToolsToolkit, DevToolsToolkitHandlers } from "./tools";
+import { makeStoreReaderLayer } from "../storeReaderService";
+import type { StoreState } from "../storeTypes";
+
+/**
+ * MCP server configuration
+ */
+export const MCP_PORT = 34438;
+export const MCP_PATH = "/mcp";
+
+/**
+ * Create the MCP server layer.
+ *
+ * This layer:
+ * 1. Creates an HTTP server on a separate port
+ * 2. Creates an MCP server at the specified path
+ * 3. Registers the DevTools toolkit with all tools
+ * 4. Provides the StoreReader service for tool handlers
+ *
+ * @param getStore - Function to get the current store state
+ */
+export const makeMcpLayer = (getStore: () => StoreState) => {
+  // Create MCP toolkit layer that registers tools with the server
+  const ToolkitLayer = McpServer.toolkit(DevToolsToolkit);
+
+  // MCP layer with toolkit registration
+  const McpLive = Layer.mergeAll(
+    // The MCP HTTP layer creates the server and registers routes on HttpRouter.Default
+    McpServer.layerHttp({
+      name: "effect-devtools",
+      version: "1.0.0",
+      path: MCP_PATH,
+    }),
+    // Register our toolkit with the server
+    ToolkitLayer,
+  ).pipe(
+    // Provide the handlers for our toolkit
+    Layer.provide(DevToolsToolkitHandlers),
+    // Provide the store reader so handlers can access state
+    Layer.provide(makeStoreReaderLayer(getStore)),
+  );
+
+  // HTTP server layer using Bun
+  const HttpServerLive = BunHttpServer.layer({ port: MCP_PORT });
+
+  // Combine everything: serve the router with MCP routes
+  return HttpRouter.Default.unwrap(
+    HttpServer.serve(HttpMiddleware.logger),
+  ).pipe(
+    HttpServer.withLogAddress,
+    Layer.provide(McpLive),
+    Layer.provide(HttpServerLive),
+  );
+};
+
+/**
+ * Run the MCP server as an Effect (for use in forked fiber)
+ */
+export const runMcpServer = (getStore: () => StoreState) =>
+  Effect.gen(function* () {
+    console.log(`[MCP] Starting MCP server on port ${MCP_PORT}...`);
+    yield* Layer.launch(makeMcpLayer(getStore));
+  });