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>

CLAUDE.md

@@ -204,7 +204,7 @@ directly loadable. Migrate incrementally, not in bulk.
 
 ```markdown
 ---
-name: skill-name-kebab-case        # letters, numbers, hyphens only
+name: skill-name-kebab-case        # clean name: dir minus the NN- prefix
 description: |
   What it does + when to use. Triggers: keyword1, keyword2, 한국어 트리거.
 ---
@@ -214,6 +214,9 @@ description: |
 Content starts here...
 ```
 
+**Naming convention:** the directory keeps its `NN-` prefix for ordering
+(`16-seo-schema-validator/`), but the skill `name:` is the **clean** form without it
+(`name: seo-schema-validator`). Names must be kebab-case and globally unique.
 Whole frontmatter ≤ 1024 chars. Full rules + migration recipe: `reference/SKILL-MIGRATION-GUIDE.md`.
 
 ## Directory Layout