Bug: Ctrl+C on the ask tool selector threw ToolAbortError, the turn
ended with stopReason === "aborted", and handleBackgroundEvent fired
sendCompletionNotification() unconditionally — producing a misleading
"Task complete" desktop toast for a turn that never actually completed.
Fix mirrors the stopReason filter already used by
#currentContextTokens, #handleMessageEnd, and the retry / TTSR /
compaction skip paths across agent-session.ts: check the most recent
assistant message via session.getLastAssistantMessage() and return
early when stopReason is "aborted" or "error".
Test coverage (event-controller-abort-guard.test.ts, 6 cases):
- aborted -> 0 sendNotification calls
- error -> 0 calls
- stop -> 1 call (normal completion)
- no last assistant message -> proceeds (defensive)
- isBackgrounded=false (foreground) -> still 0
- completion.notify=off -> still 0
Matching guard applied to the standalone desktop-notify extension
(~/.omp/agent/extensions/desktop-notify/index.ts) which is currently
the live producer of completion toasts after Phase 1 of
seed_0ca7e1143ac1.
