feat(ai-providers): add Antigravity and Cursor OAuth providers

[JS-banana]

No description provided.

[tt-a1i]

Reviewed exact head 128a1d4b9ba93339e022cd864ee504c56aa47cc3.

The Pi-native provider direction is valuable, and the focused provider tests cover substantial protocol behavior. However, the standard installed-runtime acceptance currently fails on both required Linux jobs. This is a PR-introduced release blocker, not the unrelated UI fixture failure described in the PR body.

Both Node 22.19.0 and Node 24 complete the test suites, then fail after pi install "$PWD" when pi --offline --no-session --mode rpc auto-loads the new extension:

Cannot find module .../@earendil-works/pi-ai/dist/compat.js/api/google-shared

Please use a runtime-resolvable public Pi API or vendor the required licensed conversion logic locally, and add a clean-install/runtime-load regression test that proves the published package can load the provider extension. The branch is also behind current main, so required checks must be rerun after the fix and update.

[JS-banana]

已按 review 修复，当前 head 为 6a45d6b：移除了 @earendil-works/pi-ai/api/google-shared 深层运行时依赖，将所需 MIT 许可转换逻辑本地化并补充 attribution；同时把 ai-providers 加入 packed-install extension smoke。已通过 bun run check、隔离配置下完整测试（Node 1064 passed / 1 skipped，Vitest 30/30）、source install auto-load smoke 和 packed-package explicit-load smoke。烦请复审。