feat: add GitHub stars syncing to metrics sync

- Add github.ts to npm-client with functions to fetch GitHub stars
- Add parseGitHubUrl to handle various GitHub URL formats
- Update metrics sync endpoint to fetch and store GitHub stars
- GitHub stars now factor into tool quality score calculation
- Optional GITHUB_TOKEN env var for higher API rate limits

Closes #8

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

packages/npm-client/package.json

@@ -11,6 +11,7 @@
     "./search": "./src/search.ts",
     "./package": "./src/package.ts",
     "./stats": "./src/stats.ts",
+    "./github": "./src/github.ts",
     "./rate-limiter": "./src/rate-limiter.ts"
   },
   "scripts": {