feat(gemini): add Gemini provider plugin (oauth-personal, pro/flash usage)

[Rich627]

Closes #13.

Summary

Add Gemini provider plugin with minimal first-merge scope:

• plugins/gemini/plugin.js
• plugins/gemini/plugin.json
• plugins/gemini/plugin.test.js
• plugins/gemini/icon.svg
• docs/providers/gemini.md
• README.md provider list update

Behavior:

• Supports oauth-personal
• Explicitly rejects unsupported auth modes (api-key, vertex-ai)
• Reads OAuth credentials from ~/.gemini/oauth_creds.json
• Refreshes token via Google OAuth token endpoint when needed
• Uses Gemini quota endpoints and maps to Pro / Flash lines
• Adds Account line from id_token email

Validation (local)

Environment:

• Node: v20.20.0 (via /opt/homebrew/opt/node@20/bin)

Commands run:

• npm run build ✅
• npm run test -- --run ✅

Result:

• Test Files: 38 passed
• Tests: 570 passed

---

Summary by cubic

Adds a Gemini provider plugin using personal OAuth to show Pro/Flash usage, plan tier, and account email. Fixes token refresh for Homebrew installs by discovering OAuth client credentials in Homebrew paths.

• New Features

  • Supports oauth-personal; rejects api-key and vertex-ai.
  • Uses ~/.gemini/oauth_creds.json and OAuth refresh; discovers client ID/secret from gemini-cli oauth2.js.
  • Maps plan from loadCodeAssist tier (Paid, Workspace, Free, Legacy).
  • Shows Pro and Flash usage; adds Account from id_token email.
  • Falls back to project discovery via Cloud Resource Manager; adds plugin.json, icon, docs/providers/gemini.md, README link, and tests.

• Bug Fixes

  • Restores token refresh for Gemini CLI Homebrew installs by searching oauth2.js under /opt/homebrew/opt and /usr/local/opt.

^{Written for commit fdc0d64. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 6 files

[Rich627]

Update: Homebrew path fix + real API smoke test

Bug found & fixed

Token refresh was failing for Homebrew-installed Gemini CLI because the plugin only searched ~/.bun, ~/.npm-global, and ~/.nvm paths for OAuth client credentials (oauth2.js).

Added /opt/homebrew/opt/ (Apple Silicon) and /usr/local/opt/ (Intel) Homebrew symlink paths. See commit 6bf90ba.

Smoke test results (real Gemini API)

Ran a standalone smoke test against real Gemini credentials:

[1] Auth type: oauth-personal ✅
[2] OAuth creds loaded ✅
[3] id_token email decoded ✅
[4] Token expired → refresh via Homebrew path → success ✅
[5] loadCodeAssist HTTP 200 ✅
    project: warm-transmitter-j3lhb
[6] retrieveUserQuota HTTP 200 ✅
    12 quota buckets returned:
    - gemini-2.0-flash, gemini-2.5-flash, gemini-2.5-flash-lite
    - gemini-2.5-pro, gemini-3-flash-preview, gemini-3-pro-preview
    (+ vertex variants)

Validation

• vitest --run: 38 files / 570 tests passed ✅
• tsc && vite build: passed ✅

[robinebers]

Thank you! @cubic-dev-ai review

[cubic-dev-ai]

Thank you! @cubic-dev-ai review

@robinebers I have started the AI code review. It will take a few minutes to complete.

[cubic-dev-ai]

No issues found across 6 files

[davidarny]

LGTM!

[validatedev]

LGTM!