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/src/config.py

@@ -17,6 +17,9 @@ class Settings(BaseSettings):
     gitea_webhook_secret: str
     gitea_url: str = "https://git.mosaicstack.dev"
 
+    # Anthropic API
+    anthropic_api_key: str
+
     # Server Configuration
     host: str = "0.0.0.0"
     port: int = 8000