fix(ci): install jq in the container so the Trope IR test actually runs

.github/workflows/ci.yml

@@ -27,17 +27,24 @@ jobs:
       - name: Checkout
         uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
 
-      # The pinned idris2-pack image ships no xz binary, so setup-zig's
-      # .tar.xz download fails with "tar (child): xz: Cannot exec".
-      - name: Install xz (needed to unpack the Zig tarball)
+      # The pinned idris2-pack image is minimal. Two things are missing:
+      #   xz — setup-zig's .tar.xz download dies with "tar: xz: Cannot exec"
+      #   jq — tests/check-examples.sh exits 2 with "jq is required", so the
+      #        Trope IR conformance test never actually ran in CI
+      - name: Install container prerequisites (xz, jq)
         shell: bash
         run: |
           set -euo pipefail
-          if ! command -v xz >/dev/null 2>&1; then
+          missing=()
+          command -v xz >/dev/null 2>&1 || missing+=(xz-utils)
+          command -v jq >/dev/null 2>&1 || missing+=(jq)
+          if [ "${#missing[@]}" -gt 0 ]; then
+            echo "installing: ${missing[*]}"
             apt-get update -qq
-            apt-get install -y -qq --no-install-recommends xz-utils
+            apt-get install -y -qq --no-install-recommends "${missing[@]}"
           fi
           xz --version | head -1
+          jq --version
 
       - name: Set up Zig
         uses: goto-bus-stop/setup-zig@abea47f85e598557f500fa1fd2ab7464fcb39406 # v2.2.1