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, 18 insertions, 0 deletions

diff --git a/packages/todo/src/format.test.ts b/packages/todo/src/format.test.ts
new file mode 100644
index 0000000..bffb8a3
--- /dev/null
+++ b/packages/todo/src/format.test.ts
@@ -0,0 +1,18 @@
+import { describe, expect, it } from "vitest";
+import { formatTodoResult, type TodoItem } from "./pure.js";
+
+describe("formatTodoResult", () => {
+	it("formatTodoResult: returns JSON string of the todos", () => {
+		const todos: TodoItem[] = [
+			{ content: "alpha", status: "in_progress" },
+			{ content: "beta", status: "pending" },
+		];
+		expect(formatTodoResult(todos)).toBe(JSON.stringify(todos, null, 2));
+		// spot-check it is pretty-printed JSON (indented key)
+		expect(formatTodoResult(todos)).toContain('"content": "alpha"');
+	});
+
+	it('formatTodoResult: empty array returns "[]"', () => {
+		expect(formatTodoResult([])).toBe("[]");
+	});
+});