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/.env.example

@@ -0,0 +1,13 @@
+# Gitea Configuration
+GITEA_WEBHOOK_SECRET=your-webhook-secret-here
+GITEA_URL=https://git.mosaicstack.dev
+
+# Anthropic API (for issue parsing)
+ANTHROPIC_API_KEY=sk-ant-your-api-key-here
+
+# Server Configuration
+HOST=0.0.0.0
+PORT=8000
+
+# Logging
+LOG_LEVEL=info