feat: add skills repository generators

[jongio]

Description

Adds portable repository generation for skill marketplaces and catalogs. The branch includes the create-skills-repo and create-skill workflows, GitHub Pages catalog generation, managed configuration, CI and eval setup, cross-host registration, strict image validation, and generated Azure GPT Image 2 thumbnails.

Testing

• Distribution tests: 20 passed
• create-skills-repo: 69 passed, 90.66% line coverage
• create-skill: 40 passed, 93.62% line coverage
• create-gh-pages-site: 77 checks passed
• Strict Vally lint: all three changed skills passed
• Eval lint: all three changed skills passed
• Astro catalog build: 13 pages built