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/tests/test_watcher.py

@@ -0,0 +1,16 @@
+"""task-watcher heartbeat payload — state=mode + mode 필드 검증."""
+import json
+
+from _shared.heartbeat import build_payload, WorkerStats
+
+
+def test_watcher_heartbeat_payload_carries_mode():
+    payload = json.loads(
+        build_payload(
+            "task-watcher", "watcher", "trading",
+            WorkerStats(), extra={"mode": "trading"},
+        )
+    )
+    assert payload["kind"] == "watcher"
+    assert payload["state"] == "trading"
+    assert payload["mode"] == "trading"