ci(pr-pipeline): fix flaky frontmatter check (grep -q SIGPIPE under pipefail)

The validate job runs with bash -e -o pipefail. `echo "$FM" | grep -q '^name:'`
lets grep exit on first match, which can SIGPIPE the echo; pipefail then reports
the pipeline non-zero and the || branch raises a false "Missing name/description".
The flagged file set varied run to run (and included files untouched by the PR)
while a fresh clone of the same commit passes the identical check. Reproduced:
the old form failed 3 of 5 local runs under the same shell flags, the herestring
form passed 5 of 5. Use herestrings so no pipe can be broken.

.gitea/workflows/pr-pipeline.yaml

@@ -43,17 +43,21 @@ jobs:
           echo "=== Prose Contract Frontmatter Validation ==="
           FAILED=0
           for f in $(find . -name "*.prose.md" -not -path "./.git/*" -not -path "./runs/*"); do
+            # NOTE: use herestrings, not `echo "$FM" | grep ...`. Under the runner's
+            # `-e -o pipefail`, `grep -q` exits on first match and can SIGPIPE the
+            # producer, making the pipeline report non-zero and raising a false
+            # "Missing name/description" whose file set varies run to run.
             FM=$(sed -n '/^---$/,/^---$/p' "$f" | sed '1d;$d')
             [ -z "$FM" ] && { echo "  ❌ $f: No YAML frontmatter"; FAILED=$((FAILED+1)); continue; }
 
-            KIND=$(echo "$FM" | grep '^kind:' | awk '{print $2}')
+            KIND=$(grep '^kind:' <<< "$FM" | awk '{print $2}')
             case "$KIND" in
               function|responsibility|gateway|pattern|test|template|architecture|enforcement) echo "  ✅ $f: kind=$KIND" ;;
               *) echo "  ❌ $f: Invalid kind='$KIND'"; FAILED=$((FAILED+1)) ;;
             esac
 
-            echo "$FM" | grep -q '^name:' || { echo "  ❌ $f: Missing name"; FAILED=$((FAILED+1)); }
-            echo "$FM" | grep -q '^description:' || { echo "  ❌ $f: Missing description"; FAILED=$((FAILED+1)); }
+            grep -q '^name:' <<< "$FM" || { echo "  ❌ $f: Missing name"; FAILED=$((FAILED+1)); }
+            grep -q '^description:' <<< "$FM" || { echo "  ❌ $f: Missing description"; FAILED=$((FAILED+1)); }
           done
           [ $FAILED -gt 0 ] && { echo "❌ FRONTMATTER FAILED ($FAILED error(s))"; exit 1; }
           echo "✅ Frontmatter validation passed"