fix: refresh tasks snapshot immediately after IPC task mutations

Previously, current_tasks.json was only written at container-start time,
so tasks created (or paused/cancelled/updated) during a session were
invisible to list_tasks until the next invocation.

Add an onTasksChanged callback to IpcDeps, called after every successful
mutation in processTaskIpc (schedule_task, pause_task, resume_task,
cancel_task, update_task). index.ts wires it up to write fresh snapshots
for all registered groups immediately, keeping no new coupling between
ipc.ts and the container layer.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ipc-auth.test.ts

@@ -62,6 +62,7 @@ beforeEach(() => {
     syncGroups: async () => {},
     getAvailableGroups: () => [],
     writeGroupsSnapshot: () => {},
+    onTasksChanged: () => {},
   };
 });