When a delegation completes (or fails), also write an
`activity_type='a2a_receive'` row to the caller's activity_logs so the
caller's inbox poller (workspace/inbox.py — `?type=a2a_receive`) surfaces
the result to the agent.
Why: today the only way the caller agent learns about a delegation result
is by holding open an HTTP `message/send` connection through the platform
proxy. That connection has a hard timeout (~600s) — a 90-iteration
external-runtime task on stream output routinely blows past it, and the
result emitted after the timeout lands in /dev/null. (Hongming's home
hermes hit this on 2026-05-05 — task was actively heartbeating "iteration
14/90" when the proxy timer fired.)
This PR adds the SERVER-SIDE result-push so the result is durably
delivered to the caller's inbox queue. The agent-side cutover (replace
sync httpx delegation with delegate_task_async + wait_for_message poll)
ships in the next PR — once both land, the proxy timeout class is gone.
## Feature flag
`DELEGATION_RESULT_INBOX_PUSH=1` enables the push. Default off — staging
canary first, flip after RFC #2829 PR-3 (agent-side) lands and proves
the round-trip end-to-end. With the flag off, behavior is byte-identical
to before this PR (verified by TestUpdateStatus_FlagOff_NoNewSQL).
## Two write sites
1. UpdateStatus handler (POST /workspaces/:id/delegations/:id/update)
— agent-initiated delegations report status here
2. executeDelegation goroutine — canvas-initiated delegations
(POST /workspaces/:id/delegate) report status from this background
coroutine
Both paths call `pushDelegationResultToInbox` which is best-effort: an
INSERT failure logs but does NOT propagate up. The existing
`delegate_result` row in activity_logs (the dashboard view) remains
authoritative; the new `a2a_receive` row is purely additive for the
inbox-poller to surface.
## Coverage
6 new tests in delegation_inbox_push_test.go:
- flag off → no SQL fired (the rollout-safety contract)
- flag on, completed → a2a_receive row with status=ok
- flag on, failed → a2a_receive row with status=error + error_detail
- UpdateStatus end-to-end (flag on, completed)
- UpdateStatus end-to-end (flag on, failed)
- UpdateStatus end-to-end (flag off, byte-identical to pre-PR behavior)
All 30 existing delegation_test.go tests still pass — flag default off
keeps the strict-sqlmock surface unchanged.
Refs RFC #2829.
ae79b9e9fe