Simplify spec-drift body construction

- Capture the diff directly in the existing `if ! diff …` (errexit is
  already exempt inside `if`), removing the separate `-q` pre-check,
  the intermediate pretty-printed tempfiles, and the `|| true` workaround.
- Replace the multi-echo body wrapper with two `printf`s and a 1-line
  bash `&&` truncation marker.
- Pass `--label vendored --label upstream` to `diff` so the issue body
  shows meaningful headers instead of `/dev/fd/63`.

Author: splch

.github/workflows/spec-drift.yml

@@ -22,40 +22,27 @@ jobs:
       - name: Check for drift
         id: drift
         run: |
-          python3 -m json.tool --sort-keys openapi.json > /tmp/vendored-spec.json
-          python3 -m json.tool --sort-keys /tmp/latest-spec.json > /tmp/upstream-spec.json
-          if diff -q /tmp/vendored-spec.json /tmp/upstream-spec.json > /dev/null 2>&1; then
-            exit 0
+          if ! diff -u --label vendored --label upstream <(python3 -m json.tool --sort-keys openapi.json) <(python3 -m json.tool --sort-keys /tmp/latest-spec.json) > /tmp/spec.diff; then
+            echo "drifted=true" >> "$GITHUB_OUTPUT"
           fi
-          echo "drifted=true" >> "$GITHUB_OUTPUT"
-          # diff exits 1 when files differ; pipefail would trip errexit, so capture first.
-          diff -u /tmp/vendored-spec.json /tmp/upstream-spec.json > /tmp/spec.diff || true
-          {
-            echo "The spec at api.ionq.co/v0.4/api-docs has diverged from the vendored openapi.json. Fetch the new spec and regenerate the client."
-            echo
-            echo "<details><summary>Diff (vendored → upstream, pretty-printed JSON)</summary>"
-            echo
-            echo '```diff'
-            head -c 60000 /tmp/spec.diff
-            if [[ $(wc -c < /tmp/spec.diff) -gt 60000 ]]; then
-              echo
-              echo "... (diff truncated at 60000 bytes; fetch the spec to see the rest)"
-            fi
-            echo '```'
-            echo
-            echo "</details>"
-          } > /tmp/issue-body.md
       - name: Open or update issue
         if: steps.drift.outputs.drifted == 'true'
         env:
           GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: |
+          {
+            echo "The spec at api.ionq.co/v0.4/api-docs has diverged from the vendored openapi.json. Fetch the new spec and regenerate the client."
+            printf '\n<details><summary>Diff (sorted, pretty-printed JSON)</summary>\n\n```diff\n'
+            head -c 60000 /tmp/spec.diff
+            [[ $(wc -c < /tmp/spec.diff) -gt 60000 ]] && printf '\n... (truncated)\n'
+            printf '```\n</details>\n'
+          } > /tmp/body.md
           existing=$(gh issue list --label spec-drift --state open --json number --jq '.[0].number // empty')
           if [[ -z "$existing" ]]; then
             gh issue create \
               --title "OpenAPI spec has changed upstream" \
-              --body-file /tmp/issue-body.md \
+              --body-file /tmp/body.md \
               --label spec-drift
           else
-            gh issue edit "$existing" --body-file /tmp/issue-body.md
+            gh issue edit "$existing" --body-file /tmp/body.md
           fi