refactor(skills): clean skill names (strip NN- prefix from name:) — convention change

Adopt: directory keeps its NN- ordering prefix; skill `name:` is the clean form
without it (dir 16-seo-schema-validator → name: seo-schema-validator). Nicer to
invoke, matches the original desktop/SKILL.md names, still globally unique.

- 71 root SKILL.md: name: NN-foo → name: foo (flat skills + reference-curator suite).
  Plugins (mac-optimizer/multi-agent-guide/dintel-bootstrap) already clean; 95 already clean.
- scripts/migrate_skill_root.py: derive name = dirname minus NN- prefix (skill_name()).
- CLAUDE.md + SKILL-MIGRATION-GUIDE.md: document the dir-prefix / clean-name convention.

verify_skills.py: 0 name collisions across all renamed skills. (The ~/.claude/skills
symlinks were re-pointed to the clean names separately — filesystem only.)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

custom-skills/72-dintel-doc-secretary/SKILL.md

@@ -1,5 +1,5 @@
 ---
-name: 72-dintel-doc-secretary
+name: dintel-doc-secretary
 description: |
   D.intelligence Documentation Secretary. Format meeting notes, reports, proposals, and deliverables with brand-compliant templates. Triggers: "format meeting notes", "회의록 정리", "prepare deliverable", "산출물 준비", "format report", "리포트 포맷팅", "apply brand template", "브랜드 템플릿 적용", "convert to DOCX", "문서 변환", "quality check document", "문서 품질 검토", "meeting minutes", "의사록 작성", "document formatting", "문서 정리"
 version: 1.1.0