feat(executions): plumb cancel signal into agent task-runner reply (#671 defense-in-depth)

[dolho]

Summary

Follow-up to #671 (DB-level CAS guard). The minimum fix in #671 ensures a user cancel is never overwritten by a late SUCCESS at the database boundary. This ticket tackks the defense-in-depth layer the original report asked for: make the container task-runner aware that a cancel happened and surface it in the response.

Problem (residual after #671)

When the operator cancels mid-run, Claude Code currently catches the signal, emits a graceful final message, exits with code 0, and the agent task-runner reports:

```
INFO:agent_server.routers.chat:[Task] Task completed successfully
INFO: 172.28.0.5:37822 - "POST /api/task HTTP/1.1" 200 OK
```

The DB-level fix in #671 stops this from poisoning the schedule_executions row. But:

• Container logs still say "completed successfully" — confusing during incident triage.
• Agent has no signal that its work was repudiated; for agents with side effects (Slack post, sheet rows, CRM), the agent might still finish a half-baked deliverable on the next turn.
• The HTTP response carries no `status=cancelled` marker — only the gateway knows.

Asks

• Plumb cancel signal into `docker/base-image/agent_server/routers/chat.py` so the task-runner knows when the parent gateway issued a cancel.
• On detected cancel, log `[Task] Task cancelled by user` instead of `completed successfully`.
• Include `status: "cancelled"` in the `POST /api/task` reply payload so the gateway can cross-validate the DB CAS write.
• Decide whether agent should suppress the final assistant-message side effects (Slack post, sheet write) on cancel — likely yes, but needs design.
• Base-image rebuild + rollout after the change.

Related

• Cancelled executions reported as status=success in API and DB #671 — minimum fix landed (DB CAS protects the row)
• `docker/base-image/agent_server/routers/chat.py` — task endpoint + cancel handling
• `src/backend/services/task_execution_service.py` — gateway-side execution

[AndriiPasternak31]

/claim

[github-actions]

Claimed by @AndriiPasternak31. Label status-in-progress added.

Next steps:

1. Create branch: feature/679-<short-slug>
2. Implement the feature
3. Open a PR with Fixes #679