RFD: Replace Gemini CLI with Antigravity CLI

[Fluzko]

What does this PR do?

Adds an RFD proposing we drop the gemini agent and support Antigravity CLI (agy) instead.

Gemini CLI's consumer sign-in is closed — "Sign in with Google" now fails with a message from Google telling you to migrate to Antigravity. That's the path our users take, so supporting Gemini means supporting an agent most people can't log into.

Antigravity is not a rename. Skills and MCP map over cleanly (it reads the same .agents/skills/ layout we already write, and the familiar mcpServers shape), but hooks are a new file, new shape, new event names, new payload fields and a new output contract. That's why this replaces the agent rather than adding one.

Two findings from testing agy 1.1.22 that shaped the design:

• A PreToolUse hook returning {} denies the tool call. Our dispatcher returns {} whenever no plugin contributed, which is the common case — so a naive port would block every tool call. Exit codes are ignored entirely.
• There's no SessionStart event. The RFD derives one from PreInvocation plus the first sighting of a conversationId, since invocationNum resets every turn.

The RFD includes a proposed agent-details reference page and a six-step implementation plan. No code changes here.

Disclosure questions

AI disclosure.

• The AI tool authored large parts of the code

Claude did the research and drafted the RFD, including running an instrumented agy locally to verify the hook contract rather than relying on docs. Direction, scope and the design calls were mine.

Questions for reviewers.

• Is replacing Gemini the right call, or should both be supported for a transition period?
• The session-start substitute (tracking seen conversationIds) is the least conventional part of the design — worth a look.

[jackh726]

Not sure we need this (I think of RFDs about design - and this is just about updating our support).

I can merge if you think we should; otherwise, let's just merge the impl PR