Restructure skill numbering: SEO 11-30, GTM 60-69, reserve 19-28 for future skills

Renumber 12 existing skills to new ranges:
- SEO: 11→13, 12→18, 13→16, 14→17, 15→14, 16→15, 17→29, 18→30, 19→12
- GTM: 20→60, 21→61, 22→62

Update cross-references in gateway architect/builder skills, GTM guardian
README, CLAUDE.md (skill tables + directory layout), and AGENTS.md
(domain routing ranges).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

custom-skills/13-seo-on-page-audit/desktop/skill.yaml

@@ -0,0 +1,13 @@
+# Skill metadata (extracted from SKILL.md frontmatter)
+
+name: seo-on-page-audit
+description: |
+  On-page SEO analyzer for meta tags, headings, links, images, and Open Graph. Triggers: on-page SEO, meta tags, title tag, heading structure, alt text.
+
+# Optional fields
+allowed-tools:
+  - mcp__firecrawl__*
+  - mcp__perplexity__*
+  - mcp__notion__*
+
+# triggers: []  # TODO: Extract from description