feat(coding-agent): codemode and MCP

This adds codemode and MCP support to pi. Codemode (packages/codemode) runs
model-written JavaScript in a QuickJS wasm VM inside a worker. From there the
script calls pi's tools as async functions, uses a per-session store, and can
read the model catalog and run classifiers. The codemode tool in coding-agent
is compatible with Codex's exec tool, and codemode.mode works like Codex's
tool modes (on or only). Large results are written to a JSON temp file. MCP
(packages/mcp) is a standalone client for stdio and streamable HTTP servers,
with OAuth.

The core only gets general mechanisms; codemode, tool_search and MCP are
built-in extensions that use them. Other extensions can replace these
built-ins. Tools declare an exposure: direct, model-only, codemode, deferred
or hidden. prepareLoadout() lets an orchestrating tool change which tool
declarations the model sees. ctx.executeTool() runs nested calls through
validation and the tool_call/tool_result hooks. Those calls emit events with
parentToolCallId and are recorded as bounded nestedCalls on the parent
result, which compaction and HTML export use. Tools can also return
structuredContent with an outputSchema, and can report errors with isError
instead of throwing. MCP servers come from mcp.json (global, or per project
after the project is trusted) or from pi.registerMcpServer(). They are
managed with /mcp and pi mcp list|login|logout, and their runtimes load on
first use.

Closes #10040

packages/coding-agent/examples/sdk/14-codemode-mcp.ts

@@ -0,0 +1,60 @@
+/**
+ * Codemode, Tool Search, and MCP
+ *
+ * The CLI loads `codemode`, `tool_search`, and MCP as built-in extensions. SDK sessions add
+ * them to the resource loader's extension factories.
+ *
+ * `codemode` and `tool_search` are registered inactive. Enable them through the `defaultTools` setting
+ * (`tools` would also restrict the session to the named tools, which hides MCP tools), or let the
+ * MCP extension activate them: `codemode` for servers with `codemode` or `codemode-deferred` exposure,
+ * `tool_search` for servers with `deferred` exposure.
+ */
+
+import {
+	createAgentSession,
+	createCodemodeExtension,
+	createMcpExtension,
+	createToolSearchExtension,
+	DefaultResourceLoader,
+	getAgentDir,
+	SessionManager,
+	SettingsManager,
+} from "@earendil-works/pi-coding-agent";
+
+const cwd = process.cwd();
+
+const resourceLoader = new DefaultResourceLoader({
+	cwd,
+	agentDir: getAgentDir(),
+	extensionFactories: [
+		createCodemodeExtension({ mode: "on" }),
+		createToolSearchExtension(),
+		// Reads mcp.json from the agent directory and the trusted project, like the CLI.
+		createMcpExtension(),
+	],
+});
+await resourceLoader.reload();
+
+const settingsManager = SettingsManager.create(cwd);
+settingsManager.applyOverrides({ defaultTools: ["read", "bash", "edit", "write", "codemode", "tool_search"] });
+
+const { session } = await createAgentSession({
+	resourceLoader,
+	settingsManager,
+	sessionManager: SessionManager.inMemory(),
+});
+
+try {
+	// Emits session_start, which connects the MCP servers in the background.
+	await session.bindExtensions({});
+	console.log("Active tools:", session.getActiveToolNames().join(", "));
+	session.subscribe((event) => {
+		if (event.type === "message_update" && event.assistantMessageEvent.type === "text_delta") {
+			process.stdout.write(event.assistantMessageEvent.delta);
+		}
+	});
+	await session.prompt("Use codemode to count the TypeScript files in src/ and list the three largest.");
+	console.log();
+} finally {
+	session.dispose();
+}

packages/coding-agent/examples/sdk/README.md

@@ -21,6 +21,7 @@ The runtime example shows how to build a recreate function that closes over proc
 | `11-sessions.ts` | In-memory, persistent, continue, list sessions |
 | `12-full-control.ts` | Replace everything, no discovery |
 | `13-session-runtime.ts` | Manage runtime-backed session replacement |
+| `14-codemode-mcp.ts` | Add the `codemode`, `tool_search`, and MCP extensions |
 
 ## Running