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, 66 insertions, 0 deletions

diff --git a/packages/todo/src/validate.test.ts b/packages/todo/src/validate.test.ts
new file mode 100644
index 0000000..c518041
--- /dev/null
+++ b/packages/todo/src/validate.test.ts
@@ -0,0 +1,66 @@
+import { describe, expect, it } from "vitest";
+import { validateTodos } from "./pure.js";
+
+describe("validateTodos", () => {
+	it("validateTodos: accepts valid list", () => {
+		const result = validateTodos({
+			todos: [
+				{ content: "Do thing A", status: "pending" },
+				{ content: "Do thing B", status: "in_progress" },
+				{ content: "Do thing C", status: "completed" },
+			],
+		});
+		expect(result).toEqual([
+			{ content: "Do thing A", status: "pending" },
+			{ content: "Do thing B", status: "in_progress" },
+			{ content: "Do thing C", status: "completed" },
+		]);
+	});
+
+	it("validateTodos: accepts empty array (clears the list)", () => {
+		const result = validateTodos({ todos: [] });
+		expect(result).toEqual([]);
+	});
+
+	it("validateTodos: rejects invalid status", () => {
+		const result = validateTodos({
+			todos: [{ content: "x", status: "bogus" }],
+		});
+		expect(result).toHaveProperty("error");
+	});
+
+	it("validateTodos: rejects empty content", () => {
+		const empty = validateTodos({
+			todos: [{ content: "", status: "pending" }],
+		});
+		expect(empty).toHaveProperty("error");
+		const whitespace = validateTodos({
+			todos: [{ content: "   ", status: "pending" }],
+		});
+		expect(whitespace).toHaveProperty("error");
+	});
+
+	it("validateTodos: rejects non-array todos", () => {
+		const result = validateTodos({ todos: "not-an-array" });
+		expect(result).toHaveProperty("error");
+	});
+
+	it("validateTodos: rejects null/non-object args", () => {
+		expect(validateTodos(null)).toHaveProperty("error");
+		expect(validateTodos(undefined)).toHaveProperty("error");
+		expect(validateTodos("string")).toHaveProperty("error");
+		expect(validateTodos(42)).toHaveProperty("error");
+		expect(validateTodos([])).toHaveProperty("error");
+	});
+
+	it("validateTodos: does NOT enforce one in_progress (allows multiple — description guides the model)", () => {
+		const result = validateTodos({
+			todos: [
+				{ content: "a", status: "in_progress" },
+				{ content: "b", status: "in_progress" },
+			],
+		});
+		expect(Array.isArray(result)).toBe(true);
+		expect(result).toHaveLength(2);
+	});
+});