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, 47 insertions, 0 deletions

diff --git a/packages/todo/src/store.test.ts b/packages/todo/src/store.test.ts
new file mode 100644
index 0000000..92130a3
--- /dev/null
+++ b/packages/todo/src/store.test.ts
@@ -0,0 +1,47 @@
+import { describe, expect, it } from "vitest";
+import { clearTodos, getTodos, setTodos, type TodoState } from "./pure.js";
+
+describe("getTodos", () => {
+	it("getTodos: returns fresh array copy (not the live array)", () => {
+		const state: TodoState = new Map();
+		setTodos(state, "c1", [{ content: "a", status: "pending" }]);
+		const snap = getTodos(state, "c1");
+		expect(snap).toHaveLength(1);
+		// mutating the snapshot array does not affect live state
+		snap.push({ content: "evil", status: "completed" });
+		expect(getTodos(state, "c1")).toHaveLength(1);
+	});
+
+	it("getTodos: empty array for unknown conversation", () => {
+		const state: TodoState = new Map();
+		expect(getTodos(state, "nope")).toEqual([]);
+	});
+});
+
+describe("setTodos", () => {
+	it("setTodos: replaces the list and returns a copy", () => {
+		const state: TodoState = new Map();
+		setTodos(state, "c1", [{ content: "first", status: "pending" }]);
+		// replace with a different list
+		const snap = setTodos(state, "c1", [{ content: "second", status: "in_progress" }]);
+		expect(snap).toHaveLength(1);
+		const first = snap[0];
+		if (first === undefined) throw new Error("expected an item");
+		expect(first.content).toBe("second");
+		// the previous list is gone
+		expect(getTodos(state, "c1").map((t) => t.content)).toEqual(["second"]);
+	});
+});
+
+describe("clearTodos", () => {
+	it("clearTodos: deletes the conversation's list", () => {
+		const state: TodoState = new Map();
+		setTodos(state, "c1", [{ content: "x", status: "pending" }]);
+		expect(getTodos(state, "c1")).toHaveLength(1);
+		clearTodos(state, "c1");
+		expect(getTodos(state, "c1")).toEqual([]);
+		// a second clear is a no-op
+		clearTodos(state, "c1");
+		expect(getTodos(state, "c1")).toEqual([]);
+	});
+});