feat: add guarded Git and pull request tools

[nahin-fr]

Summary

Adds opt-in guarded Git and draft pull-request MCP tools for Agent and Handoff modes.

Security boundaries

• Explicit paths only; no implicit staging.
• Protected branches and destructive Git operations are blocked.
• Handoff stage/commit scope defaults to .ai-bridge/** and validates the full staged set.
• Push and PR require expected branch, expected HEAD, and confirmation.

Configuration

• Agent: CODEXPRO_GIT_WRITE, CODEXPRO_GIT_PUSH, CODEXPRO_GITHUB_PR.
• Handoff: CODEXPRO_HANDOFF_GIT_WRITE, CODEXPRO_HANDOFF_GIT_PUSH, CODEXPRO_HANDOFF_GITHUB_PR, CODEXPRO_HANDOFF_GIT_ALLOWED_PATHS.

Validation

• npm run build
• npm run smoke
• Disposable Git integration including temporary bare-remote push and mocked PR creation.
• Local Agent/Handoff services run this feature build on ports 8788/8789.

Rollback

Restore the backed-up LaunchAgent wrappers and restart both services.

No merge, force-push, reset, clean, rebase, branch deletion, remote mutation, or other destructive Git action is exposed.