refactor(skills): mandate metadata.version on every skill's frontmatter

Only 12 of 39 skills carried metadata.version, and adoption tracked
which plugin a skill lived in rather than any stated rule: core,
gitea and lint were consistent adopters, bin and kyberforge were
consistent non-adopters, git was split with one outlier. There was
no documented convention, and skill-author's own bump logic was
already written as if presence were conditional.

metadata.version is now required on every skill. The 19 skills here
that never carried one (bin, kyberforge, gitea-files) are seeded at
1.0.0, not 0.1.0 -- that value stays reserved for a skill's actual
creation point under skill-author's existing convention. The
skill-frontmatter pre-commit hook now fails a SKILL.md missing the
field, the same class of failure as a missing name/description.

Full rationale in the new ADR. The git-plugin skills that also need
this field follow in the next commit, bundled with issue #113's rtk
normalization since both touch the same files.

Refs: #127
ADR: 0022
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EeH8SCbcrCAQrtymkNuhKP

.pre-commit-config.yaml

@@ -259,8 +259,18 @@ repos:
           - |
             for f in "$@"; do
               if [[ -f "$f" ]]; then
-                if ! grep -q "^name:" "$f" || ! grep -q "^description:" "$f"; then
-                  echo "ERROR: $f is missing required frontmatter fields (name: and description:)"
+                missing=""
+                if ! grep -q "^name:" "$f"; then
+                  missing="${missing}name: "
+                fi
+                if ! grep -q "^description:" "$f"; then
+                  missing="${missing}description: "
+                fi
+                if ! grep -A10 "^metadata:" "$f" | grep -q "  version:"; then
+                  missing="${missing}metadata.version "
+                fi
+                if [[ -n "$missing" ]]; then
+                  echo "ERROR: $f is missing required frontmatter fields (${missing})"
                   exit 1
                 fi
               fi