[research] Antigravity agent support — needs reporter input before merge

[mattpocock]

Summary

• No code changed in agents.ts — this is a research capture only, so the investigation into adding Antigravity as a request-logger agent isn't lost.
• Adds request-logger/docs/antigravity-research.md: what's confirmed (Antigravity CLI honors GOOGLE_GEMINI_BASE_URL, the same env var the existing gemini → api-key provider already uses) vs. what's unverified/second-hand (whether the Antigravity IDE can be redirected through a proxy at all — one archived community project reports it can't via plain env-var/MITM/DNS).
• Not ready to merge: I need more information from whoever requested this before writing a real catalogue entry — see "Open questions for the reporter" in the doc.

Open questions (blocking a real implementation)

• IDE or CLI — which did you mean?
• Have you gotten Antigravity to route through any custom base URL/proxy successfully? What host/setting worked?
• Which model (Gemini 3 Pro vs. Claude Sonnet 4.5) were you running it against?
• Do you have a captured request we could use as a fixture?

Test plan

• Reporter confirms IDE vs. CLI and provides a working redirection method
• A real request capture is obtained and attached (or a .request.txt)
• Once confirmed, follow up with a real agents.ts entry + test, per the existing "Add an agent" contract in request-logger/README.md