feat: add manage case studies skill

[americano98]

Summary

• add a repository-scoped skill for safely adding, editing, reordering, promoting, and removing case studies
• add a dedicated validator for case-study data, assets, links, categories, testimonials, and cross-page references
• expose the skill to Claude Code through a relative symlink while keeping .agents/skills as the source of truth

Why

Case-study updates touch several independently maintained surfaces and can easily leave broken links, missing assets, invalid categories, or stale references. This skill documents the complete workflow and blocks incomplete or unsafe changes before they are applied.

Validation

• node --check scripts/validate-case-studies.js
• node scripts/validate-case-studies.js
• git diff --cached --check

[vercel]

@americano98 is attempting to deploy a commit to the neondatabase Team on Vercel.

A member of the Team first needs to authorize it.