Support Antigravity Google auth

[cbusillo]

Summary

• add Antigravity Keychain credential loading as the Google/Gemini auth fallback
• keep Gemini CLI OAuth metadata and local oauth_creds.json behavior working for existing users
• add App Store screenshot assets and update docs/setup copy for Antigravity plus Gemini CLI access

Validation

• scripts/commit-gate.sh
• JetBrains changed-file inspection: only non-actionable spelling findings for provider keys/bundle identifiers (cloudaicompanion, googleusercontent, shinycomputers, contextpanel)
• scripts/context-panel-runtime-baseline.sh install --launch
• installed app Antigravity-only proof: temporary missing Gemini OAuth path refreshed through Antigravity, Google report healthy, 8 Google limits
• restored real Gemini account proof: Google report healthy, 8 Google limits
• final scripts/context-panel-runtime-baseline.sh check: baseline=OK app=/Applications/Context Panel.app