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/README.md

@@ -10,10 +10,13 @@ The coordinator receives webhook events from Gitea when issues are assigned, una
 
 - HMAC SHA256 signature verification
 - Event routing (assigned, unassigned, closed)
+- AI-powered issue metadata parsing (using Anthropic Sonnet)
+- Context estimation and agent assignment
+- Dependency tracking (blocks/blocked_by)
 - Comprehensive logging
 - Health check endpoint
 - Docker containerized
-- 85%+ test coverage
+- 95%+ test coverage
 
 ## Development
 
@@ -44,9 +47,11 @@ ruff check src/
 ### Running locally
 
 ```bash
-# Set environment variables
-export GITEA_WEBHOOK_SECRET="your-secret-here"
-export LOG_LEVEL="info"
+# Copy environment template
+cp .env.example .env
+
+# Edit .env with your values
+# GITEA_WEBHOOK_SECRET, GITEA_URL, ANTHROPIC_API_KEY
 
 # Run server
 uvicorn src.main:app --reload --port 8000
@@ -82,6 +87,7 @@ Health check endpoint.
 | ---------------------- | ------------------------------------------- | -------- | ------- |
 | `GITEA_WEBHOOK_SECRET` | Secret for HMAC signature verification      | Yes      | -       |
 | `GITEA_URL`            | Gitea instance URL                          | Yes      | -       |
+| `ANTHROPIC_API_KEY`    | Anthropic API key for issue parsing         | Yes      | -       |
 | `LOG_LEVEL`            | Logging level (debug, info, warning, error) | No       | info    |
 | `HOST`                 | Server host                                 | No       | 0.0.0.0 |
 | `PORT`                 | Server port                                 | No       | 8000    |
@@ -96,6 +102,7 @@ docker build -t mosaic-coordinator .
 docker run -p 8000:8000 \
   -e GITEA_WEBHOOK_SECRET="your-secret" \
   -e GITEA_URL="https://git.mosaicstack.dev" \
+  -e ANTHROPIC_API_KEY="your-anthropic-key" \
   mosaic-coordinator
 ```
 
@@ -120,15 +127,21 @@ pytest -v
 ```
 apps/coordinator/
 ├── src/
-│   ├── main.py          # FastAPI application
-│   ├── webhook.py       # Webhook endpoint handlers
-│   ├── security.py      # HMAC signature verification
-│   └── config.py        # Configuration management
+│   ├── main.py             # FastAPI application
+│   ├── webhook.py          # Webhook endpoint handlers
+│   ├── parser.py           # Issue metadata parser (Anthropic)
+│   ├── models.py           # Data models
+│   ├── security.py         # HMAC signature verification
+│   ├── config.py           # Configuration management
+│   └── context_monitor.py  # Context usage monitoring
 ├── tests/
 │   ├── test_security.py
 │   ├── test_webhook.py
-│   └── conftest.py      # Pytest fixtures
-├── pyproject.toml       # Project metadata & dependencies
+│   ├── test_parser.py
+│   ├── test_context_monitor.py
+│   └── conftest.py         # Pytest fixtures
+├── pyproject.toml          # Project metadata & dependencies
+├── .env.example            # Environment variable template
 ├── Dockerfile
 └── README.md
 ```