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/src/config.py

@@ -27,6 +27,11 @@ class Settings(BaseSettings):
     # Logging
     log_level: str = "info"
 
+    # Coordinator Configuration
+    coordinator_poll_interval: float = 5.0
+    coordinator_max_concurrent_agents: int = 10
+    coordinator_enabled: bool = True
+
 
 def get_settings() -> Settings:
     """Get settings instance (lazy loaded)."""