feat(#160): Implement basic orchestration loop

Implements the Coordinator class with main orchestration loop:
- Async loop architecture with configurable poll interval
- process_queue() method gets next ready issue and spawns agent (stub)
- Graceful shutdown handling with stop() method
- Error handling that allows loop to continue after failures
- Logging for all actions (start, stop, processing, errors)
- Integration with QueueManager from #159
- Active agent tracking for future agent management

Configuration settings added:
- COORDINATOR_POLL_INTERVAL (default: 5.0s)
- COORDINATOR_MAX_CONCURRENT_AGENTS (default: 10)
- COORDINATOR_ENABLED (default: true)

Tests: 27 new tests covering all acceptance criteria
Coverage: 92% overall (100% for coordinator.py)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/coordinator/tests/conftest.py

@@ -112,8 +112,9 @@ def client(webhook_secret: str, gitea_url: str, monkeypatch: pytest.MonkeyPatch)
     monkeypatch.setenv("LOG_LEVEL", "debug")
 
     # Force reload of settings
-    from src import config
     import importlib
+
+    from src import config
     importlib.reload(config)
 
     # Import app after settings are configured