Salvage follow-up on top of @pmos69's #29474. The PR resolved the
Antigravity OAuth client purely by discovering it from an installed `agy`
binary or HERMES_ANTIGRAVITY_CLIENT_ID/SECRET env vars, so users without
agy installed hit a hard 'client ID not available' error.
Antigravity's desktop OAuth client is a public, non-confidential installed-app
client (PKCE provides the security), baked into every copy of the Antigravity
CLI — same posture as the gemini-cli credentials Hermes already ships in
google_oauth.py. Bake it in as the final fallback (env -> discovery -> public
default) and add the public default Code Assist project as the discovery
fallback, matching the reference Antigravity flow. Now consumers can
authenticate directly without agy installed.
