feat(#158): Implement issue parser agent

Add AI-powered issue metadata parser using Anthropic Sonnet model.
- Parse issue markdown to extract: estimated_context, difficulty,
  assigned_agent, blocks, blocked_by
- Implement in-memory caching to avoid duplicate API calls
- Graceful fallback to defaults on parse failures
- Add comprehensive test suite (9 test cases)
- 95% test coverage (exceeds 85% requirement)
- Add ANTHROPIC_API_KEY to config
- Update documentation and add .env.example

Fixes #158

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

apps/coordinator/tests/conftest.py

@@ -108,6 +108,7 @@ def client(webhook_secret: str, gitea_url: str, monkeypatch: pytest.MonkeyPatch)
     # Set test environment variables
     monkeypatch.setenv("GITEA_WEBHOOK_SECRET", webhook_secret)
     monkeypatch.setenv("GITEA_URL", gitea_url)
+    monkeypatch.setenv("ANTHROPIC_API_KEY", "test-anthropic-api-key")
     monkeypatch.setenv("LOG_LEVEL", "debug")
 
     # Force reload of settings