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/60-gtm-audit/SKILL.md

@@ -1,5 +1,5 @@
 ---
-name: 60-gtm-audit
+name: gtm-audit
 description: |
   GTM container audit using Chrome DevTools and DTM Agent for tag verification.
   Triggers: audit GTM, GTM analysis, tag debugging, dataLayer inspection.