feat(task-watcher): heartbeat 발신 (state=mode, paused 이유 노출)

- watcher_loop 에서 mode 판정 직후 worker:task-watcher:heartbeat SET EX 45
- payload: build_payload(state=mode, extra={"mode": mode})
- LOOP_INTERVAL 30s < TTL 45s → 만료 전 주기적 갱신
- conftest.py 추가: services/ 를 sys.path에 주입해 _shared import 가능
- tests/test_watcher.py: payload kind/state/mode 필드 검증 (1 passed)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

services/task-watcher/conftest.py

@@ -0,0 +1,5 @@
+"""Make services/ root importable so `from _shared.heartbeat import ...` works during tests."""
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))