feat(surfaces): todo task list sidebar view

Add a dedicated "Tasks" sidebar view for the per-conversation todo surface
(model-maintained via todo_write tool; read-only, conversation-scoped).

- parseTodoPayload: pure parser for the rendererId: "todo" custom field
  (TodoItem { content, status, priority } — types defined FE-side, not in wire)
- TodoList.svelte: renders the task list with status indicators (spinner for
  in_progress, checkmark for completed, X for cancelled, empty circle for
  pending) + priority dots (red/yellow/gray)
- SurfaceView dispatches rendererId: "todo" to TodoList
- App.svelte: "Tasks" view kind (always visible; "No tasks yet" empty state),
  todo surface pulled out of the generic Extensions list, re-mounts per
  conversation via {#key}

681 tests green.

1 files changed, 58 insertions, 0 deletions

diff --git a/src/features/surface-host/logic/todo.ts b/src/features/surface-host/logic/todo.ts
new file mode 100644
index 0000000..b8e027b
--- /dev/null
+++ b/src/features/surface-host/logic/todo.ts
@@ -0,0 +1,58 @@
+/**
+ * Pure parser for the `rendererId: "todo"` custom-field payload.
+ *
+ * The `todo` backend extension maintains a per-conversation task list (written
+ * by the model via a `todo_write` tool) and exposes it as a read-only
+ * conversation-scoped surface with one `custom` field
+ * (`rendererId: "todo"`, `payload: TodoPayload`). This parser validates the
+ * untyped `payload: unknown` at the network seam so a hostile/partial payload
+ * can never crash the renderer (graceful skip → null).
+ *
+ * The `TodoItem` type is NOT in `@dispatch/wire` — it is defined by the `todo`
+ * extension and carried in the surface payload, so we define it here (the FE's
+ * rendering contract for the shape). Empty `todos` is a valid, parseable state
+ * (the model hasn't created a list / cleared it); the caller hides the panel.
+ */
+export type TodoStatus = "pending" | "in_progress" | "completed" | "cancelled";
+export type TodoPriority = "high" | "medium" | "low";
+
+export interface TodoItem {
+	readonly content: string;
+	readonly status: TodoStatus;
+	readonly priority: TodoPriority;
+}
+
+export interface TodoData {
+	readonly todos: readonly TodoItem[];
+}
+
+const STATUSES = new Set<string>(["pending", "in_progress", "completed", "cancelled"]);
+const PRIORITIES = new Set<string>(["high", "medium", "low"]);
+
+function isTodoItem(v: unknown): v is TodoItem {
+	if (typeof v !== "object" || v === null) return false;
+	const o = v as Record<string, unknown>;
+	return (
+		typeof o.content === "string" &&
+		typeof o.status === "string" &&
+		STATUSES.has(o.status) &&
+		typeof o.priority === "string" &&
+		PRIORITIES.has(o.priority)
+	);
+}
+
+export function parseTodoPayload(payload: unknown): TodoData | null {
+	if (typeof payload !== "object" || payload === null) return null;
+	const obj = payload as Record<string, unknown>;
+	const raw = obj.todos;
+	if (!Array.isArray(raw)) return null;
+	const todos: TodoItem[] = [];
+	for (const entry of raw) {
+		if (!isTodoItem(entry)) return null;
+		todos.push(entry);
+	}
+	return { todos };
+}
+
+/** The `rendererId` the `todo` extension's `custom` surface field uses. */
+export const TODO_RENDERER_ID = "todo";