Add Antigravity CLI fallback to Antigravity provider

[rohithgoud30]

Summary

• add Antigravity CLI (agy) as a fallback inside the existing antigravity provider instead of introducing a separate provider
• keep the probe order aligned with the review feedback from Add Antigravity CLI provider #477: local language server first, app SQLite/OAuth Cloud Code fallback second, CLI keychain/Cloud Code fallback last
• try Antigravity IDE SQLite credentials before legacy Antigravity credentials, and try all app SQLite token candidates so stale credentials in one DB do not hide valid credentials in another
• support account-scoped keychain reads with host.keychain.readGenericPassword(service, account?)
• document the IDE/App/CLI data sources and why Antigravity should not show guessed Claude/Codex-style token/cost history

Closes #475.

Why

#477 added an antigravity-cli provider, but the review feedback was to consolidate this into one Antigravity provider because the IDE, app, and CLI share the same quota pools. Gemini stays unchanged for now.

This keeps the UI simple: one Antigravity entry, same existing quota rows, more fallback paths when the standalone app or CLI is the only authenticated source.

Notes

I kept plugins/antigravity/plugin.json unchanged, so this does not add new links, rows, or visual layout changes. The provider still shows quota only; local Antigravity transcripts/logs do not expose reliable token/cost accounting.

Manual verification with only the standalone /Applications/Antigravity.app installed returned:

Plan: Google AI Pro
Gemini Pro: 0% used
Gemini Flash: 0% used
Claude: 0% used

Validation

• git diff --check
• bun run build
• CI=1 bun run test (1118 tests passed)
• cargo test --manifest-path src-tauri/Cargo.toml keychain
• cargo test --manifest-path src-tauri/Cargo.toml redact_log_message_redacts_account_and_paths

[cubic-dev-ai]

1 issue found across 5 files

<sub>Reply with feedback, questions, or to request a fix.

Fix all with cubic | Re-trigger cubic</sub>