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>

install.sh

@@ -17,8 +17,8 @@
 #   /plugin install skills@mathias
 #
 # That path carries the hooks and MCP server definitions too, which symlinks
-# cannot. This script covers the harnesses with no plugin system: Crush, Pi
-# and OpenCode.
+# cannot. This script covers the harnesses with no plugin system: Crush, Pi,
+# OpenCode, and Antigravity.
 #
 # Wiring model — one managed directory, one link per harness:
 #
@@ -27,6 +27,13 @@
 #   ~/.config/crush/skills      -> ~/.agents/skills
 #   ~/.config/opencode/skills   -> ~/.agents/skills
 #
+# Antigravity has no symlink-directory discovery — it reads a path-entries
+# JSON instead (~/.gemini/config/skills.json:
+# {"entries":[{"path":"..."}]}). One entry points at ~/.agents/skills, so it
+# sees exactly the same surfaces-filtered, collision-resolved set every other
+# harness does. Requires jq; skipped with a warning if jq is unavailable —
+# every other target in this script stays dependency-free on purpose.
+#
 # ~/.agents/skills was itself a link to this checkout until #27. That encoded
 # an assumption nobody had stated — that this checkout is the ONLY source of
 # skills on the host — and on flamingo the only way to install this library
@@ -53,6 +60,7 @@ CHECKOUT_DIR="${SKILLS_CHECKOUT_DIR:-$HOME/.local/share/skills}"
 AGENTS_DIR="${AGENTS_SKILLS_DIR:-$HOME/.agents/skills}"
 CRUSH_DIR="${CRUSH_SKILLS_DIR:-$HOME/.config/crush/skills}"
 OPENCODE_DIR="${OPENCODE_SKILLS_DIR:-$HOME/.config/opencode/skills}"
+ANTIGRAVITY_SKILLS_JSON="${ANTIGRAVITY_SKILLS_JSON:-$HOME/.gemini/config/skills.json}"
 EXTRA_SOURCES="${SKILLS_EXTRA_SOURCES:-}"
 # Recorded beside the managed directory rather than inside it: a stray file in
 # there would look like a skill to every harness reading the dir.
@@ -213,6 +221,42 @@ link_dir() {
   log "linked $link → $target"
 }
 
+# wire_antigravity — register $AGENTS_DIR in Antigravity's skills.json.
+# Merges rather than overwrites: skills.json is a documented user-editable
+# config (json_configs.md), so a hand-added entry (e.g. a personal skills
+# dir) must survive a re-run untouched.
+wire_antigravity() {
+  if ! command -v jq >/dev/null 2>&1; then
+    log "Antigravity: jq not found — skipping (needed to merge $ANTIGRAVITY_SKILLS_JSON safely)"
+    return
+  fi
+
+  local existing already updated
+  mkdir -p "$(dirname "$ANTIGRAVITY_SKILLS_JSON")"
+
+  if [ -s "$ANTIGRAVITY_SKILLS_JSON" ]; then
+    existing="$(cat "$ANTIGRAVITY_SKILLS_JSON")"
+  else
+    existing='{}'
+  fi
+
+  already="$(printf '%s' "$existing" | jq --arg p "$AGENTS_DIR" \
+    '[.entries // [] | .[] | select(.path == $p)] | length' 2>/dev/null)" || {
+    log "Antigravity: $ANTIGRAVITY_SKILLS_JSON is not valid JSON — leaving it untouched"
+    return 1
+  }
+
+  if [ "$already" -gt 0 ]; then
+    log "Antigravity: $AGENTS_DIR already registered in $ANTIGRAVITY_SKILLS_JSON"
+    return
+  fi
+
+  updated="$(printf '%s' "$existing" | jq --arg p "$AGENTS_DIR" \
+    '.entries = ((.entries // []) + [{path: $p}])')"
+  printf '%s\n' "$updated" > "$ANTIGRAVITY_SKILLS_JSON"
+  log "Antigravity: registered $AGENTS_DIR in $ANTIGRAVITY_SKILLS_JSON"
+}
+
 main() {
   ensure_checkout
 
@@ -224,6 +268,7 @@ main() {
 
   link_dir "$CRUSH_DIR" "$AGENTS_DIR"
   link_dir "$OPENCODE_DIR" "$AGENTS_DIR"
+  wire_antigravity
 
   log "done — $LINKED_COUNT skill(s) linked from $(sources | wc -l | tr -d ' ') source(s), ref=$REF"
   log "Claude Code: install the plugin instead — /plugin marketplace add $REPO_URL"