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/pyproject.toml

@@ -9,6 +9,7 @@ dependencies = [
     "pydantic>=2.5.0",
     "pydantic-settings>=2.1.0",
     "python-dotenv>=1.0.0",
+    "anthropic>=0.39.0",
 ]
 
 [project.optional-dependencies]