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

.pre-commit-config.yaml

@@ -238,87 +238,6 @@ repos:
         pass_filenames: false
         always_run: true
 
-      - id: skill-frontmatter
-        stages: ['pre-commit']
-        name: SKILL.md frontmatter validation
-        description: Ensure SKILL.md files have required frontmatter fields
-        entry: bash
-        language: system
-        files: '^plugins/[^/]+/\.apm/skills/[^/]+/SKILL\.md$'
-        # Pinned by tests/test-skill-frontmatter.sh, which drives this exact
-        # `bash -c <script> <arg0> <files...>` call shape rather than a copy of
-        # the script -- the bug below was invisible to any test that did not.
-        args:
-          - -c
-          - |
-            # Every check reads the FRONTMATTER only, never the whole file. A
-            # `metadata:` / `name:` / `description:` line inside a body code
-            # fence is documentation (skill-author quotes exactly such a block)
-            # and used to satisfy these greps.
-            for f in "$@"; do
-              [[ -f "$f" ]] || continue
-
-              fm="$(awk '
-                { sub(/\r$/, "") }
-                NR == 1 { sub(/^\357\273\277/, "") }
-                !opened && /^[[:blank:]]*$/ { next }
-                !opened {
-                  if ($0 ~ /^---[[:blank:]]*$/) { opened = 1; next }
-                  exit
-                }
-                /^---[[:blank:]]*$/ { closed = 1; exit }
-                { print }
-                END { if (!opened || !closed) exit 3 }
-              ' "$f")" || {
-                echo "ERROR: $f has no closing YAML frontmatter block (expected --- ... --- at the top of the file)"
-                exit 1
-              }
-
-              missing=""
-              printf '%s\n' "$fm" | grep -q "^name:" || missing="${missing}name: "
-              printf '%s\n' "$fm" | grep -q "^description:" || missing="${missing}description: "
-
-              # Scoped to the `metadata:` block and stopped at the next
-              # top-level key, so a `version:` under a following `source:` list
-              # cannot stand in for it; the `^  version:` anchor is exact, so a
-              # deeper-nested `    version:` cannot either. No line budget, so a
-              # long `metadata:` block does not hide the key.
-              ver="$(printf '%s\n' "$fm" | awk '
-                /^metadata:/ { inm = 1; next }
-                inm && /^[A-Za-z]/ { exit }
-                inm && /^  version:/ {
-                  v = $0
-                  sub(/^  version:[[:blank:]]*/, "", v)
-                  sub(/[[:blank:]]+#.*$/, "", v)
-                  sub(/[[:blank:]]+$/, "", v)
-                  print "found:" v
-                  exit
-                }
-              ')"
-              [[ -n "$ver" ]] || missing="${missing}metadata.version "
-
-              if [[ -n "$missing" ]]; then
-                echo "ERROR: $f is missing required frontmatter fields (${missing})"
-                exit 1
-              fi
-
-              raw="${ver#found:}"
-              v="$raw"
-              case "$v" in
-                \"*\") v="${v#\"}"; v="${v%\"}" ;;
-                \'*\') v="${v#\'}"; v="${v%\'}" ;;
-              esac
-              if [[ ! "$v" =~ ^[0-9]+\.[0-9]+\.[0-9]+$ ]]; then
-                echo "ERROR: $f has a malformed frontmatter metadata.version (${raw:-<empty>}) -- expected a three-part semver, e.g. \"1.0.0\""
-                exit 1
-              fi
-            done
-          # arg0 for `bash -c`. WITHOUT it pre-commit's first filename lands in
-          # $0 and is dropped from "$@" -- so a single-file commit, the normal
-          # case, ran the loop zero times and reported Passed having checked
-          # nothing. Do not remove; tests/test-skill-frontmatter.sh pins it.
-          - skill-frontmatter
-
       - id: skill-size-check
         stages: ['pre-commit']
         name: SKILL.md size and context-budget ceilings