Bump to v2.1.0: changelog, self-deleting cleanup for deprecated skills

- Version bump across package.json, plugin.json, marketplace.json
- Changelog entry for v2.1 in index.html
- Hero version link updated
- Added <post-update-cleanup> section to impeccable SKILL.md that
  detects and removes leftover files from renamed/merged skills
  (arrange, normalize, onboard, extract, frontend-design,
  teach-impeccable). Verifies files contain "impeccable" before
  deleting to avoid touching unrelated user skills. Self-deletes
  after first run so it only executes once per update.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "impeccable",
   "description": "Design vocabulary and skills for frontend development. Includes 18 commands (/polish, /distill, /audit, /typeset, /overdrive, etc.) and an enhanced impeccable skill with curated anti-patterns.",
-  "version": "2.0.7",
+  "version": "2.1.0",
   "author": {
     "name": "Paul Bakaus",
     "email": "paul@paulbakaus.com"