- Create gh-cli utility module with auth detection and command execution
- Refactor GitHubClient to use gh CLI commands with REST API fallback
- Refactor PR Monitor to use gh CLI for PR operations
- Add listIssues() and getIssue() methods to GitHubClient
- Remove in-app GitHubRateLimiter (gh CLI handles rate limiting)
- Update all tests for gh CLI implementation
- Update AGENTS.md and extension docs for gh CLI auth preference
