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/test_security.py

@@ -3,8 +3,6 @@
 import hmac
 import json
 
-import pytest
-
 
 class TestSignatureVerification:
     """Test suite for HMAC SHA256 signature verification."""