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/16-seo-schema-validator/code/scripts/requirements.txt

@@ -0,0 +1,9 @@
+# 13-seo-schema-validator dependencies
+extruct>=0.16.0
+jsonschema>=4.21.0
+rdflib>=7.0.0
+lxml>=5.1.0
+beautifulsoup4>=4.12.0
+requests>=2.31.0
+python-dotenv>=1.0.0
+rich>=13.7.0