feat: bundle provider auth plugins

Co-authored-by: ItzR3NO <ItzR3NO@users.noreply.github.com>

extensions/google-antigravity-auth/README.md

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+# Google Antigravity Auth (Clawdbot plugin)
+
+OAuth provider plugin for **Google Antigravity** (Cloud Code Assist).
+
+## Enable
+
+Bundled plugins are disabled by default. Enable this one:
+
+```bash
+clawdbot plugins enable google-antigravity-auth
+```
+
+Restart the Gateway after enabling.
+
+## Authenticate
+
+```bash
+clawdbot models auth login --provider google-antigravity --set-default
+```
+
+## Notes
+
+- Antigravity uses Google Cloud project quotas.
+- If requests fail, ensure Gemini for Google Cloud is enabled.