Add API-key auth option for the Gemini consult lane (follow-up to #778) #965

Follow-up to #778 (Gemini CLI → Antigravity CLI migration).

#778 implements the Gemini consult lane via the Antigravity CLI (agy), which authenticates OAuth / Google Sign-In only. Verified against the official antigravity-cli docs (README "Authentication") and agy --help: there is no API-key path — no GEMINI_API_KEY/GOOGLE_API_KEY env var, no --key flag, no auth subcommand (CHANGELOG confirms "OAuth token persistence"). First-run OAuth is interactive and cannot run headless in CI.

This issue tracks adding an API-key-based auth option for the Gemini perspective, for environments where OAuth/agy doesn't fit (headless CI, no Google subscription) — likely via a Gemini Developer API backend (the "Approach A" deferred in #778), offered alongside the agy OAuth lane.

Tradeoff: per-token API is ~3× the subscription cost for our volume (per #778's analysis), so this would be an opt-in alternative, not the default.

Deferred — not blocking #778.