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-vale-hooks-consumer.sh

@@ -77,6 +77,8 @@ name: demo
 description: >
   Use when the caller wants a demonstration skill $skill_body across two
   physical lines of one folded block scalar.
+metadata:
+  version: "1.0.0"
 ---
 
 Body.