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>

reference/SKILL-MIGRATION-GUIDE.md

@@ -51,7 +51,7 @@ description: |                     # third person; START with "Use when…" / wh
 ---
 ```
 
-- [ ] `name`: kebab-case, no spaces/parens/special chars. (May keep the `NN-` prefix.)
+- [ ] `name`: the **clean** skill name = directory name **minus its `NN-` ordering prefix** (dir `16-seo-schema-validator` → `name: seo-schema-validator`). kebab-case, no spaces/parens/special chars. Dirs keep the number prefix for ordering; the skill `name` never carries it.
 - [ ] `description`: third-person; says **when to use** + trigger keywords (EN + KO).
 - [ ] **Whole frontmatter ≤ 1024 characters.**
 - [ ] Body: concise markdown (aim < 500 words of always-on content); push detail into