chore: fold skill-frontmatter into skill-size-check

skill-frontmatter was a 62-line bash script inlined in
.pre-commit-config.yaml, re-parsing SKILL.md frontmatter with grep and
awk to check for name/description/metadata.version fields.
skill-size-check.sh already parses the same frontmatter block with
PyYAML for its ADR-0020 checks, so the two checks belonged in one
script.

Adds a ~20-line required-frontmatter check (name, description,
metadata.version as three-part semver) to scripts/skill-size-check.sh.
Removes the inline skill-frontmatter hook from .pre-commit-config.yaml
and deletes tests/test-skill-frontmatter.sh (366 lines). Removes the
79-line "the other hook on that scope" discussion from
docs/spec/gates.md and its now-dangling cross-reference, replacing
both with a one-line note of the fold, and updates the pre-push hook
counts there.

Updates fixture builders in test-skill-size-check.sh,
test-adr0020-body-checks.sh, test-adr0020-targets.sh,
test-adr0020-differential.sh, and test-vale-hooks-consumer.sh to carry
valid metadata.version so the new check doesn't spuriously fail
existing fixtures that predate it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YR2CjVumUbEGWcMikcoXBD

tests/test-skill-size-check.sh

@@ -35,6 +35,8 @@ make_fixture() {
     echo "---"
     echo "name: $name"
     echo "description: Test fixture."
+    echo "metadata:"
+    echo "  version: \"1.0.0\""
     echo "---"
     for ((i = 1; i <= lines; i++)); do
       w=""
@@ -173,9 +175,11 @@ make_line_fixture() {
     echo "---"
     echo "name: $name"
     echo "description: Test fixture."
+    echo "metadata:"
+    echo "  version: \"1.0.0\""
     echo "---"
   } > "$file"
-  body_lines=$((total_lines - 4))
+  body_lines=$((total_lines - 6))
   for ((i = 1; i <= body_lines; i++)); do
     echo "word"
   done >> "$file"
@@ -228,6 +232,8 @@ make_word_fixture() {
     echo "---"
     echo "name: $name"
     echo "description: Test fixture."
+    echo "metadata:"
+    echo "  version: \"1.0.0\""
     echo "notes:$padding"
     echo "---"
     echo ""
@@ -241,6 +247,8 @@ make_word_fixture() {
     echo "---"
     echo "name: $name"
     echo "description: Test fixture."
+    echo "metadata:"
+    echo "  version: \"1.0.0\""
     echo "notes:$padding"
     echo "---"
     echo ""
@@ -287,6 +295,8 @@ make_budget_fixture() {
     echo "---"
     echo "name: $name"
     echo "description: $desc"
+    echo "metadata:"
+    echo "  version: \"1.0.0\""
     echo "---"
     echo ""
     python3 -c "print(' '.join(['word'] * $body_words))"
@@ -352,6 +362,8 @@ make_tree_fixture() {
       echo "---"
       echo "name: $(basename "$sib")"
       echo "description: Use when doing the other thing. Do not use for anything else."
+      echo "metadata:"
+      echo "  version: \"1.0.0\""
       echo "---"
       echo ""
       echo "Do the thing."
@@ -361,6 +373,8 @@ make_tree_fixture() {
     echo "---"
     echo "name: $label"
     echo "description: $desc"
+    echo "metadata:"
+    echo "  version: \"1.0.0\""
     echo "---"
     echo ""
     python3 -c "print(' '.join(['word'] * $body_words))"
@@ -518,6 +532,8 @@ make_hand_invoked_fixture() {
     echo "name: $name"
     echo "description: $desc"
     echo "disable-model-invocation: true"
+    echo "metadata:"
+    echo "  version: \"1.0.0\""
     echo "---"
     echo ""
     python3 -c "print(' '.join(['word'] * $body_words))"
@@ -577,6 +593,8 @@ HAND_FALSE="$TMPDIR/hand-false.md"
   echo "name: hand-false"
   echo "description: $HAND_DESC"
   echo "disable-model-invocation: false"
+  echo "metadata:"
+  echo "  version: \"1.0.0\""
   echo "---"
   echo ""
   echo "Do the thing."
@@ -851,6 +869,8 @@ cat > "$LOCALE_SKILL/SKILL.md" <<'LOCALEEOF'
 ---
 name: locale-skill
 description: A valid skill description that is well within the limit.
+metadata:
+  version: "1.0.0"
 ---
 
 ## Step 1