fix(test): stop the Antigravity cases failing on a host without jq

main has been red since e9ad692. The gate passed; the shell suite failed on
three Antigravity cases.

Not a bug in the wiring. install.sh treats jq as optional by design — every
other target is dependency-free, so wire_antigravity warns and skips when jq is
missing. It did exactly that on the runner:

  [skills] Antigravity: jq not found — skipping

The tests were the problem: they inherited whatever jq the host happened to
have. koala has it and reported 12/12; the runner does not and reported 3
failures on the same commit. Works-on-my-machine, caught by CI doing its job.

Two changes:

- The three merge-behaviour cases now run only where jq exists, and print a SKIP
  naming what is not covered. A case that fails because the host lacks an
  optional dependency is a false red, and false reds are how a suite stops being
  read.
- The skip path gets a real test of its own, because it is a contract rather than
  a fallback. install.sh gains SKILLS_JQ so the jq-absent branch is exercisable
  on a host that has jq — the same seam pattern as DOCTOR_LATEST_TAG_CMD. It
  asserts no skills.json is written, the skip is reported, and the other
  harnesses still wire.

Verified both ways on koala by building a jq-free PATH:
  with jq:    install: 13 passed, 0 failed, 0 skipped
  without jq: install: 10 passed, 0 failed, 3 skipped

Consequence worth naming: CI now SKIPS the merge assertions rather than running
them, so idempotency and entry-preservation are untested there until the runner
gets jq. Filed as #42.

Refs #41
Bump-Type: patch

install.sh

@@ -62,6 +62,10 @@ 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="${SKILLS_EXTRA_SOURCES:-}"
+# Overridable so the jq-absent branch is testable on a host that has jq. That
+# branch is a contract, not a fallback: jq is optional here while every other
+# target in this script is dependency-free.
+JQ="${SKILLS_JQ:-jq}"
 # 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.
 SOURCES_MANIFEST="$(dirname "$AGENTS_DIR")/$(basename "$AGENTS_DIR")-sources"
@@ -226,7 +230,7 @@ link_dir() {
 # 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
+  if ! command -v "$JQ" >/dev/null 2>&1; then
     log "Antigravity: jq not found — skipping (needed to merge $ANTIGRAVITY_SKILLS_JSON safely)"
     return
   fi
@@ -240,7 +244,7 @@ wire_antigravity() {
     existing='{}'
   fi
 
-  already="$(printf '%s' "$existing" | jq --arg p "$AGENTS_DIR" \
+  already="$(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
@@ -251,7 +255,7 @@ wire_antigravity() {
     return
   fi
 
-  updated="$(printf '%s' "$existing" | jq --arg p "$AGENTS_DIR" \
+  updated="$(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"