Support using kicad-happy as a Google Antigravity plugin

[ademuri]

This adds a plugin.json to configure this repo as an Antigravity plugin, and updates the documentation to support this.

The Gemini CLI is deprecated, and Antigravity is the recommended replacement.

I tested that this validates and installs successfully with Antigravity 1.1.11.

[aklofas]

Merged, thanks! This also fixes #38 (the dangling kidoc symlink was breaking the GitHub Action download for all v2 consumers), so good timing. I pushed one small commit to your branch pre-merge updating the two remaining "works with … Gemini CLI" mentions (README intro, llms.txt) to Antigravity.

One question from the tier-precedence doc: Antigravity's workspace tier reads .agents/plugins/ — this repo already has .agents/plugins/marketplace.json there for Codex. Since your install test passed, agy presumably tolerates it, but if you ever see it choke on that file, please flag it.

The root plugin.json version is now part of the release version-sync set, so it'll bump with the other manifests each release.

[ademuri]

Sounds good, I'll let you know if I see an issue with .agents/plugins/marketplace.json. Thanks!