feat(todo): per-conversation task list tool + surface

New standard tool extension with a single todo_write tool (opencode
todowrite pattern: full-list replace, returns JSON, no business-rule
enforcement — the description guides the model). Per-conversation in-memory
state + per-conversation surface (rendererId: todo, scope: conversation)
via subscriber-notify (message-queue pattern).

Wave 0 (kernel contract): added conversationId?: string to ToolExecuteContext
(additive, backward-compatible). Wired in dispatch.ts — the kernel already
had it but wasn't passing it through to tools.

Wave 1 (todo extension): pure core (validateTodos — shape only; getTodos/
setTodos/clearTodos; buildTodoSpec; formatTodoResult). Shell:
createTodoWriteTool + surface provider. Tool description matches opencode's
todowrite.txt depth (when-to-use, examples, task states). Priority field
removed (bloats the tool with little value). 25 tests.

Wave 2 (host-bin): registered todo in CORE_EXTENSIONS + dep + root tsconfig ref.

Verified: tsc EXIT 0, 1123 vitest, biome clean (314 files). Boot smoke clean.
FE handoff: frontend-todo-handoff.md.

1 files changed, 81 insertions, 0 deletions

diff --git a/packages/todo/src/extension.ts b/packages/todo/src/extension.ts
new file mode 100644
index 0000000..80af7aa
--- /dev/null
+++ b/packages/todo/src/extension.ts
@@ -0,0 +1,81 @@
+/**
+ * todo extension — owns the per-conversation task list (state + surface + the
+ * `todo_write` tool). Plugs into the kernel host via a manifest +
+ * `activate(host)`; the model maintains the list via the tool, and the surface
+ * renders the live list for the frontend (subscriber-notify, same pattern as
+ * message-queue). State is in-memory (per-process, per-conversation) — no
+ * persistence, mirroring message-queue.
+ */
+
+import type { Extension, HostAPI, Manifest } from "@dispatch/kernel";
+import type { SurfaceContext, SurfaceProvider } from "@dispatch/surface-registry";
+import { surfaceRegistryHandle } from "@dispatch/surface-registry";
+import type { SurfaceSpec } from "@dispatch/ui-contract";
+import { buildTodoSpec, getTodos, TODO_SURFACE_ID, type TodoState } from "./pure.js";
+import { createTodoWriteTool } from "./tool.js";
+
+export const manifest: Manifest = {
+	id: "todo",
+	name: "Todo Tool",
+	version: "0.0.0",
+	apiVersion: "^0.1.0",
+	trust: "bundled",
+	activation: "eager",
+	dependsOn: ["surface-registry"],
+	capabilities: {},
+	contributes: {
+		tools: ["todo_write"],
+	},
+};
+
+export function activate(host: HostAPI): void {
+	const registry = host.getService(surfaceRegistryHandle);
+
+	const state: TodoState = new Map();
+	const subscribers = new Set<() => void>();
+
+	function notify(): void {
+		for (const sub of subscribers) {
+			sub();
+		}
+	}
+
+	host.defineTool(createTodoWriteTool({ state, notify }));
+
+	function getSpec(context?: SurfaceContext): SurfaceSpec {
+		const convId = context?.conversationId;
+		const todos = convId === undefined ? [] : getTodos(state, convId);
+		return buildTodoSpec(todos);
+	}
+
+	function invoke(_actionId: string, _payload?: unknown, _context?: SurfaceContext): void {
+		// The todo surface is read-only: the model mutates the list via the
+		// `todo_write` tool; no client-facing surface actions.
+	}
+
+	const provider: SurfaceProvider = {
+		catalogEntry: {
+			id: TODO_SURFACE_ID,
+			region: "side",
+			title: "Tasks",
+			scope: "conversation",
+		},
+		getSpec,
+		invoke,
+		subscribe(onChange) {
+			subscribers.add(onChange);
+			return () => {
+				subscribers.delete(onChange);
+			};
+		},
+	};
+
+	registry.register(provider);
+
+	host.logger.info("todo: registered");
+}
+
+export const extension: Extension = {
+	manifest,
+	activate,
+};