feat: visibility and control over background subagents (list, open, cancel)

feat: visibility and control over background subagents (list, open, cancel)

Feature description

When the agent dispatches background subagents (via delegate_task), there is
no way to see them, open the underlying chat, or cancel a runaway one. They
run invisibly in the background.

Proposed solution

• Backend:
  • GET /api/chats/{id}/subagents and POST /api/chats/{id}/subagents/{delegation_id}/cancel (ownership-checked like the rest of the chat router).
  • Registry helpers to track cancel_async_subagent and a parent_chat_id filter, plus a socket push (e.g. chat:subagents) on state changes.
  • Foreground delegate_task returning the same JSON shape as background dispatches (delegation_id + subagent_chat_id) so every delegation is clickable.
• Frontend:
  • A "Subagents" bar above the chat input (tasks-card style) listing running/finished background subagents with Open (opens the subagent chat in a tab) and Kill (confirm, then cancel API) actions.
  • Live updates via the existing events:chat socket channel, same pattern as chat:tasks.
  • Expanded delegate_task tool calls that carry a subagent chat id get an Open button; the background completion row gets an "Open sub-agent chat" action.

Additional context

Implemented on a fork; can share implementation notes.

Start by tracing the delegate_task entry point, chat router, subagent registry, and existing events:chat/chat:tasks socket flow. Map the backend ownership checks and frontend tasks-card behavior before implementing the proposed list, open, cancel, and live-update interactions. Done means background subagents are visible, openable, cancellable, and foreground and completion rows expose their chats consistently.