feat: add Antigravity CLI support, --list flag, and skill descriptions

- Replace Gemini CLI with Antigravity CLI throughout (--gemini kept as
  hidden fallback flag; GEMINI_SKILLS_DIR honoured as legacy env var)
- Default install dir updated to ~/.gemini/antigravity-cli/skills
- Add ANTIGRAVITY_SKILLS_DIR env var with GEMINI_SKILLS_DIR fallback
- Add -l/--list flag: lists available skills with one-line descriptions
- Add descriptions.json mapping skill names to short descriptions;
  fetch_descriptions() prefers a local copy when running from a clone,
  falls back to remote URL for curl-pipe installs
- Document ship-it skill in README (was missing since its addition)
- Fix Manual Install cd path: claude-skills → ai-skills

descriptions.json

@@ -0,0 +1,6 @@
+{
+  "git-publish-workflow": "Automate the full lifecycle from uncommitted local work to an open Pull Request, with built-in quality gates.",
+  "readme-sync-audit": "Keep README.md accurate by programmatically aligning it with the current state of the codebase.",
+  "ship-it": "Run a comprehensive pre-flight audit and publish changes to a new PR in a single command.",
+  "systematic-enumeration": "Eliminate counting and membership errors via explicit set enumeration and atomic element testing."
+}