feat(todo): port opencode's declarative whole-list todo tool

Replace the imperative id-based CRUD todo tool (add/update/list/get/remove)
with opencode's declarative whole-list design: a single `todos` param that
replaces the entire list each call. No model-visible ids, no delta reasoning,
no "task not found" spirals.

- core: TaskItem { id, content, status }; statuses pending|in_progress|
  completed|cancelled. TaskList.setTasks/getTasks/onChange. New rich
  TODO_DESCRIPTION adapted from opencode's todowrite.txt.
- api: TASK_MANAGEMENT_GUIDANCE system-prompt section (from anthropic.txt);
  updated TOOL_DESCRIPTIONS.todo. Reload fix: TabStatusSnapshot now carries
  per-tab tasks so getAllStatuses rehydrates the panel on reconnect.
- frontend: mirror types; hydrate tasks from snapshot in both restore paths;
  upgrade sidebar Tasks panel to render content + all four statuses + progress.
- tests: new core task-list.test.ts (15); updated api TaskList mocks +
  getAllStatuses task-snapshot coverage.

bun run check clean; 569 tests pass; all packages typecheck.

1 files changed, 158 insertions, 0 deletions

diff --git a/packages/core/tests/tools/task-list.test.ts b/packages/core/tests/tools/task-list.test.ts
new file mode 100644
index 0000000..5903fec
--- /dev/null
+++ b/packages/core/tests/tools/task-list.test.ts
@@ -0,0 +1,158 @@
+import { describe, expect, it, vi } from "vitest";
+import { createTaskListTool, TaskList } from "../../src/tools/task-list.js";
+import type { TaskItem } from "../../src/types/index.js";
+
+describe("TaskList (declarative store)", () => {
+	it("starts empty", () => {
+		const list = new TaskList();
+		expect(list.getTasks()).toEqual([]);
+	});
+
+	it("setTasks replaces the whole list and assigns positional ids", () => {
+		const list = new TaskList();
+		const result = list.setTasks([
+			{ content: "first", status: "in_progress" },
+			{ content: "second", status: "pending" },
+		]);
+		expect(result).toEqual([
+			{ id: "task-1", content: "first", status: "in_progress" },
+			{ id: "task-2", content: "second", status: "pending" },
+		]);
+		expect(list.getTasks()).toEqual(result);
+	});
+
+	it("a second setTasks fully replaces the previous list (no append)", () => {
+		const list = new TaskList();
+		list.setTasks([
+			{ content: "a", status: "completed" },
+			{ content: "b", status: "completed" },
+			{ content: "c", status: "pending" },
+		]);
+		const next = list.setTasks([{ content: "only", status: "in_progress" }]);
+		expect(next).toEqual([{ id: "task-1", content: "only", status: "in_progress" }]);
+		expect(list.getTasks()).toHaveLength(1);
+	});
+
+	it("preserves all four statuses", () => {
+		const list = new TaskList();
+		const result = list.setTasks([
+			{ content: "p", status: "pending" },
+			{ content: "i", status: "in_progress" },
+			{ content: "c", status: "completed" },
+			{ content: "x", status: "cancelled" },
+		]);
+		expect(result.map((t) => t.status)).toEqual([
+			"pending",
+			"in_progress",
+			"completed",
+			"cancelled",
+		]);
+	});
+
+	it("defaults missing/invalid status to pending", () => {
+		const list = new TaskList();
+		const result = list.setTasks([
+			{ content: "no status" },
+			{ content: "bogus", status: "done" },
+			{ content: "junk", status: 42 },
+		]);
+		expect(result.map((t) => t.status)).toEqual(["pending", "pending", "pending"]);
+	});
+
+	it("an empty array clears the list", () => {
+		const list = new TaskList();
+		list.setTasks([{ content: "x", status: "pending" }]);
+		expect(list.setTasks([])).toEqual([]);
+		expect(list.getTasks()).toEqual([]);
+	});
+
+	it("getTasks returns copies (no external mutation leaks in)", () => {
+		const list = new TaskList();
+		list.setTasks([{ content: "x", status: "pending" }]);
+		const snapshot = list.getTasks();
+		snapshot[0].content = "mutated";
+		expect(list.getTasks()[0].content).toBe("x");
+	});
+
+	it("onChange fires on every setTasks with the new snapshot", () => {
+		const list = new TaskList();
+		const seen: TaskItem[][] = [];
+		const unsubscribe = list.onChange((tasks) => seen.push(tasks));
+		list.setTasks([{ content: "a", status: "pending" }]);
+		list.setTasks([{ content: "b", status: "completed" }]);
+		expect(seen).toHaveLength(2);
+		expect(seen[0]).toEqual([{ id: "task-1", content: "a", status: "pending" }]);
+		expect(seen[1]).toEqual([{ id: "task-1", content: "b", status: "completed" }]);
+		unsubscribe();
+		list.setTasks([{ content: "c", status: "pending" }]);
+		expect(seen).toHaveLength(2);
+	});
+});
+
+describe("createTaskListTool", () => {
+	it("exposes a single declarative `todos` parameter and the name `todo`", () => {
+		const tool = createTaskListTool(new TaskList());
+		expect(tool.name).toBe("todo");
+		// One top-level param: the whole-list `todos` array.
+		const shape = (tool.parameters as { shape: Record<string, unknown> }).shape;
+		expect(Object.keys(shape)).toEqual(["todos"]);
+	});
+
+	it("execute updates the store and echoes the list WITHOUT ids", async () => {
+		const list = new TaskList();
+		const tool = createTaskListTool(list);
+		const out = await tool.execute({
+			todos: [
+				{ content: "plan", status: "completed" },
+				{ content: "build", status: "in_progress" },
+			],
+		});
+		expect(JSON.parse(out)).toEqual([
+			{ content: "plan", status: "completed" },
+			{ content: "build", status: "in_progress" },
+		]);
+		// Store has ids; the echo does not.
+		expect(list.getTasks()).toEqual([
+			{ id: "task-1", content: "plan", status: "completed" },
+			{ id: "task-2", content: "build", status: "in_progress" },
+		]);
+	});
+
+	it("execute fires onChange so the UI broadcast is wired", async () => {
+		const list = new TaskList();
+		const cb = vi.fn();
+		list.onChange(cb);
+		const tool = createTaskListTool(list);
+		await tool.execute({ todos: [{ content: "x", status: "pending" }] });
+		expect(cb).toHaveBeenCalledTimes(1);
+	});
+
+	it("execute with an empty array clears the store", async () => {
+		const list = new TaskList();
+		list.setTasks([{ content: "x", status: "pending" }]);
+		const tool = createTaskListTool(list);
+		const out = await tool.execute({ todos: [] });
+		expect(JSON.parse(out)).toEqual([]);
+		expect(list.getTasks()).toEqual([]);
+	});
+
+	it("execute defaults invalid status to pending in both store and echo", async () => {
+		const list = new TaskList();
+		const tool = createTaskListTool(list);
+		const out = await tool.execute({ todos: [{ content: "x", status: "done" }] });
+		expect(JSON.parse(out)).toEqual([{ content: "x", status: "pending" }]);
+		expect(list.getTasks()[0].status).toBe("pending");
+	});
+
+	it("execute rejects a non-array todos param", async () => {
+		const tool = createTaskListTool(new TaskList());
+		const out = await tool.execute({ todos: "nope" });
+		expect(out).toMatch(/Error/);
+	});
+
+	it("execute rejects items missing a content string", async () => {
+		const tool = createTaskListTool(new TaskList());
+		const out = await tool.execute({ todos: [{ status: "pending" }] });
+		expect(out).toMatch(/Error/);
+	});
+});