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-adr0020-targets.sh

@@ -61,6 +61,8 @@ write_skill() {
     echo "---"
     echo "name: $2"
     echo "description: $3"
+    echo "metadata:"
+    echo "  version: \"1.0.0\""
     echo "---"
     echo ""
     echo "Do the thing."