Adds a skills-manager skill so an agent can discover and install catalog skills from inside an Antigravity session, backed by a Python standard-library CLI (`skills_cli.py`) with no third-party dependencies, and a root `plugin.json` manifest.
The engine resolves its install destination the same way `bin/cli.js` does: skills land in `~/.gemini/antigravity/skills`, overridable via `AG_SKILLS_DIR` with a leading `~` expanded, so the two CLIs always agree on where skills live. Installs replace the target folder rather than merging into it, and a partial or failed install exits non-zero instead of reporting soft success.
A catalog entry's path is validated against the exact shape `build-catalog.js` emits before it is used as a filesystem path or appended to the pinned GitHub URLs, and contents-API responses are held to the same rule. Search matches on each whitespace-separated term, and filtered listings are returned in full.
The Python suite runs in CI via `npm run test:python`, and `plugin.json` and `skills_cli.py` are included in the published package.
The `agy plugin install` path is documented as experimental pending end-to-end verification against a released `agy` CLI. Also includes two Windows correctness fixes: path-separator normalization in `build-catalog.js` and CRLF handling in `check-catalog-drift.js`.
tonicofonico