fix(ui-web): a running sub-agent has no stop control on the page, though the backend exposes one #497

Summary

The page shows the sub-agents a conversation has running -- the desk's agents lane, the roster, the instance surface -- and none of those surfaces can stop one. The backend already exposes the verbs: subagent.cancel_instance { session_key, agent, handle } stops the spawns behind one rendered (agent, handle) pair, subagent.interrupt { subagent_id } stops one spawn by id, and subagent.cancel_session { session_key } sweeps the session. ui-web/src calls none of them; the TUI's agents overlay does (ui-tui/src/components/agentsOverlay.tsx, the interrupt helper posts subagent.interrupt).

Steps to reproduce

1. Open a session and have the main agent spawn two sub-agents (or run a DAG with two nodes).
2. Open the Subagents page or the desk's agents lane while both are running.
3. Look for a way to stop one of them without stopping the other.

Expected behavior

Each running instance row offers a stop, the way the TUI overlay does, wired to subagent.cancel_instance for the (agent, handle) the row already holds.

Actual behavior

There is no stop on any instance row. The only stop on the page is the composer's go button, which is the main agent's turn.cancel; with the fix that pairs it with cancel_by_session it stops every spawn of the session at once, which is the right meaning for that button and still leaves "stop this one" with no control.

Two related but distinct gaps are filed separately:

• a direct instance chat cannot be stopped because the global stop sends turn.cancel without a target -- A direct sub-agent chat cannot be stopped: no cancel on the instance surface, and the global stop omits the target #459;
• the go button goes inert while a delegated result re-enters the conversation -- fix(ui-web): the page's stop goes inert while a delegated result re-enters the conversation #496.

Waiting on the ui-web/ freeze to lift; the fix is a row control under ui-web/src/features/subagents/ (and the desk's agents lane) calling an RPC the generated client already carries -- subagent.cancel_instance is in ui-web/src/rpc/generated.ts today.

Environment

• Raven main at 6d8c363 (2026-09-18), web page served by raven web
• macOS 15, Apple Silicon

Logs or screenshots

N/A -- the defect is a missing control.