feat(install): re-add Antigravity as a wiring target

Antigravity discovers skills via a path-entries JSON
(~/.gemini/config/skills.json), not a symlink directory. One entry points
at $AGENTS_DIR — the same managed, surfaces-filtered, collision-resolved
directory Crush/OpenCode/Pi already read — so Antigravity sees exactly the
same skill set as every other harness, for free.

Requires jq; every other target in this script stays dependency-free on
purpose, so wire_antigravity skips with a warning (not a hard failure) if
jq is missing. Merges into skills.json rather than overwriting it: the
file is documented as user-editable (json_configs.md — e.g. a personal
skills dir), so a hand-added entry must survive a re-run.

Was previously removed as "unused" (mathias/skills#41 — Antigravity usage
picked back up). Mistral Vibe stays removed.

Closes #41

Bump-Type: minor

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -100,9 +100,11 @@ default to point into `~/dev`.
 | `AGENTS_SKILLS_DIR` | `$HOME/.agents/skills` |
 | `CRUSH_SKILLS_DIR` | `$HOME/.config/crush/skills` |
 | `OPENCODE_SKILLS_DIR` | `$HOME/.config/opencode/skills` |
+| `ANTIGRAVITY_SKILLS_JSON` | `$HOME/.gemini/config/skills.json` (registers `AGENTS_SKILLS_DIR` as a path entry; needs `jq`) |
 | `SKILLS_EXTRA_SOURCES` | unset (colon-separated extra source directories) |
 
-Antigravity and Mistral Vibe targets were removed — unused.
+The Mistral Vibe target was removed — unused. Antigravity's was removed for
+the same reason and re-added once it came back into use.
 
 ## Checking a host