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>

scripts/migrate_skill_root.py

@@ -4,7 +4,8 @@ migrate_skill_root.py — additive bulk migration: give each skill a root SKILL.
 
 Non-destructive. For every skill directory under custom-skills/ that lacks a root
 SKILL.md, it generates one by copying the skill's existing directive and:
-  - setting `name:` to the directory name (guaranteed kebab-case + unique),
+  - setting `name:` to the directory name MINUS its NN- ordering prefix
+    (dir `16-seo-schema-validator` -> name `seo-schema-validator`; clean + unique),
   - preserving `description` and the markdown body verbatim,
   - validating the result (frontmatter <= 1024 chars, kebab name, description present).
 
@@ -29,6 +30,11 @@ FM_RE = re.compile(r"^---\n(.*?)\n---\n?(.*)$", re.S)
 MAX_FM = 1024
 
 
+def skill_name(dirname):
+    """Skill `name:` = directory name without its NN- ordering prefix."""
+    return re.sub(r"^\d+-", "", dirname)
+
+
 def is_skill_dir(d):
     """A skill dir is a real skill, not shared infra."""
     if not d.is_dir():
@@ -106,7 +112,7 @@ def main(argv=None):
             rows.append((name, "MANUAL", "no desktop/ or code/ SKILL.md source (commands/README only)"))
             manual += 1
             continue
-        text, issues = build_root_skill(src.read_text(encoding="utf-8"), d.name)
+        text, issues = build_root_skill(src.read_text(encoding="utf-8"), skill_name(d.name))
         rel = src.relative_to(d)
         if text is None:
             rows.append((name, "MANUAL", f"{rel}: {issues[0]}"))