perf(kyberforge): scope skill-frontmatter to .apm/ and parallelize plugin sync

skill-frontmatter's files: pattern matched any SKILL.md, so it ran twice per
sync -- once on the .apm/ source and again on the compiled flat-mirror copy.
Scoped it to .apm/skills/ like its sibling hooks (skill-size-check,
vale-audit-prefilter-skill), which already only check the source.

sync-plugin-content.sh ran `apm pack` once per plugin, serially -- each
invocation is dominated by fixed CLI startup cost rather than per-plugin
work, so 6 plugins paid that cost 6 times over (~3.1s). Backgrounds the
per-plugin work instead, buffering each plugin's output so concurrent
DRIFT/FAIL messages can't interleave, then flushes in stable order after
`wait` (~1.3s, confirmed idempotent on a real sync).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01X7GvKuJfy2WrdBmUttV4DT

.pre-commit-config.yaml

@@ -149,7 +149,7 @@ repos:
         description: Ensure SKILL.md files have required frontmatter fields
         entry: bash
         language: system
-        files: 'SKILL\.md$'
+        files: '^plugins/[^/]+/\.apm/skills/[^/]+/SKILL\.md$'
         args:
           - -c
           - |