feat(commands): add /seo-auditor — 7-phase SEO audit pipeline for documentation

- 7 phases: discovery → meta tags → content quality → keywords → links → sitemap → report
- Integrates 8 marketing-skill scripts: seo_checker, content_scorer,
  humanizer_scorer, headline_scorer, seo_optimizer, sitemap_analyzer,
  schema_validator, topic_cluster_mapper
- References 6 SEO knowledge bases for audit framework, AI search,
  content optimization, URL design, internal linking, AI detection
- Auto-fixes: generic titles, missing descriptions, broken links, orphan pages
- Preserves high-ranking pages — only fixes critical issues on those
- Registered in both commands/ (distributable) and .claude/commands/ (local)

Also: sync all doc counts — 28 plugins, 26 eng-core skills, 21 commands

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

docs/getting-started.md

@@ -140,7 +140,7 @@ Choose your platform and follow the steps:
 
 | Bundle | Install Command | Skills |
 |--------|----------------|--------|
-| **Engineering Core** | `/plugin install engineering-skills@claude-code-skills` | 25 |
+| **Engineering Core** | `/plugin install engineering-skills@claude-code-skills` | 26 |
 | **Engineering POWERFUL** | `/plugin install engineering-advanced-skills@claude-code-skills` | 30 |
 | **Product** | `/plugin install product-skills@claude-code-skills` | 14 |
 | **Marketing** | `/plugin install marketing-skills@claude-code-skills` | 43 |