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>

tests/install_test.sh

@@ -55,6 +55,7 @@ run_install() {
     AGENTS_SKILLS_DIR="$_f/home/.agents/skills" \
     CRUSH_SKILLS_DIR="$_f/home/.config/crush/skills" \
     OPENCODE_SKILLS_DIR="$_f/home/.config/opencode/skills" \
+    ANTIGRAVITY_SKILLS_JSON="$_f/home/.gemini/config/skills.json" \
     SKILLS_EXTRA_SOURCES="$_extra" \
     bash "$SUT" 2>&1)
   rc=$?
@@ -183,5 +184,53 @@ else
   no 'a source with no surfaces convention is linked as-is' "$out"
 fi
 
+# --- 9. Antigravity: the managed agents dir is registered, not the raw checkout
+# skills.json entries must point at $AGENTS_DIR — that's what already carries
+# surfaces-filtering, collision handling and multi-source merge. Pointing at
+# the checkout directly would bypass all of it and give Antigravity a
+# different skill set than every other harness.
+f=$(new_fixture antigravity)
+out=$(run_install "$f")
+AG="$f/home/.gemini/config/skills.json"
+if [ ! -f "$AG" ]; then
+  no 'Antigravity skills.json is created' "$out"
+elif ! command -v jq >/dev/null 2>&1; then
+  no 'jq available to verify Antigravity wiring' "(jq missing on test host)"
+else
+  registered=$(jq -r --arg p "$f/home/.agents/skills" '[.entries[]? | select(.path == $p)] | length' "$AG")
+  if [ "$registered" = "1" ]; then
+    ok 'Antigravity registers the managed agents dir, not the raw checkout'
+  else
+    no 'Antigravity registers the managed agents dir, not the raw checkout' "$(cat "$AG")"
+  fi
+fi
+
+# --- 10. Antigravity wiring is idempotent ------------------------------------
+f=$(new_fixture antigravity_idempotent)
+run_install "$f" >/dev/null
+out=$(run_install "$f")
+AG="$f/home/.gemini/config/skills.json"
+count=$(jq '.entries | length' "$AG" 2>/dev/null || echo "?")
+if [ "$count" = "1" ]; then
+  ok 'Antigravity wiring is idempotent (no duplicate entries)'
+else
+  no 'Antigravity wiring is idempotent (no duplicate entries)' "entries=$count"$'\n'"$out"
+fi
+
+# --- 11. Antigravity wiring preserves entries it doesn't own -----------------
+# A user may hand-edit skills.json to point at their own personal skills dir
+# (json_configs.md documents this). Our merge must not clobber it.
+f=$(new_fixture antigravity_preserve)
+mkdir -p "$f/home/.gemini/config"
+printf '{"entries":[{"path":"/Users/mathias/personal-skills"}]}' > "$f/home/.gemini/config/skills.json"
+out=$(run_install "$f")
+AG="$f/home/.gemini/config/skills.json"
+personal=$(jq -r '[.entries[]? | select(.path == "/Users/mathias/personal-skills")] | length' "$AG" 2>/dev/null || echo 0)
+if [ "$personal" = "1" ]; then
+  ok 'Antigravity wiring preserves pre-existing hand-authored entries'
+else
+  no 'Antigravity wiring preserves pre-existing hand-authored entries' "$(cat "$AG")"$'\n'"$out"
+fi
+
 printf '\ninstall: %d passed, %d failed\n' "$pass" "$fail"
 [ "$fail" -eq 0 ]