Setup Antigravity Devcontainer

Adds Antigravity to a devcontainer already generated by the setup-devcontainer skill. This skill never touches devcontainer.json or the Dockerfile — it appends its own marker-keyed content to post-create.sh, post-start.sh (if the skills link or skill sync is opted into), and README.md, and idempotently ensures its own Network Manifest entry exists in .devcontainer/network-manifest.json.

0. Security & Trust

This skill's generated automation touches three areas an external audit may flag. Each is opt-in, scoped to this container, and exists for a specific reason — none of it runs on the host, only inside the generated devcontainer.