- Added gh_pr_checkout tool to check out GitHub pull requests into git worktrees with branch metadata.
- Added gh_pr_push tool to push checked-out pull request branches to source fork with force-with-lease support.
- Implemented 15+ git helper functions for worktree, branch, and remote management operations.
- Extended GhPrViewData interface with headRefOid, headRepository, isCrossRepository, and maintainerCanModify fields.
- Added comprehensive integration tests for gh_pr_checkout and gh_pr_push tools with fixture setup and cleanup.