chore: automated code review using antigravity

[rickeylev]

Free gemini code assist is being disabled in a few weeks, and I've found the ai
reviews useful enough to want to keep them, so replace it with a custom agent
based upon the antigravity sdk that does code reviews.

I've added a Gemini API key to the project for it to use. It's free-tier
level quota, so is relatively limited, hence reviews are limited to project
maintainer PRsor when a maintainer comments /review.

[gemini-code-assist]

Code Review

This pull request refactors and simplifies the release tool by removing several subcommands (such as add-backports, complete-sync-changelog, and on-pr-merged), simplifying the backport processing logic, and introducing an AI-assisted code review bot (antigravity_review.py). Feedback on these changes highlights several critical runtime issues: a TypeError in promote_rc.py (and its tests) due to an invalid remote_ref keyword argument in get_commit_sha, a NameError in utils.py because argparse is raised without being imported, a potential ValueError in release_issue.py when parsing non-numeric PR references, and a potential regex substitution error in changelog_news.py if news entries contain backslashes.

Important

The consumer version of Gemini Code Assist on GitHub is being sunset. Starting June 18, 2026, new organization installations will be blocked, and all code review activity will officially cease on July 17, 2026.
For more details on the timeline and next steps, please review the Help Documentation.