refactor: rename skills for consistent prefix conventions

[raiderrobert]

Summary

• Rename 5 skills so related skills share a prefix and names are self-describing
• unsafe-checker → rust-unsafe (join rust-* convention)
• frontend-design-enhanced → design-frontend (drop orphaned -enhanced, add design-* prefix)
• anti-slop-audit → design-audit (add design-* prefix)
• visual-design-preflight → design-preflight (add design-* prefix)
• find-references → cite-check (disambiguate from LSP "find references")
• Update all internal cross-references (routing table in rust/SKILL.md, learning path in rust/ref/anti-patterns.md)

Result

skills/
  rust/              # hub router
  rust-domain/       # domain-specific Rust
  rust-lsp/          # Rust code analysis via LSP
  rust-unsafe/       # unsafe Rust / FFI review
  design-frontend/   # build web components/pages
  design-audit/      # audit designs for AI slop
  design-preflight/  # pre-check before Pencil MCP work
  diecut/            # diecut template scaffolding
  cite-check/        # find original sources for claims/quotes

Test plan

• ls skills/ shows 9 directories with new names, no old names
• grep -r for old names under skills/ returns zero hits
• Each renamed SKILL.md has correct name: frontmatter
• skills/rust/SKILL.md routing table points to rust-unsafe
• skills/rust/ref/anti-patterns.md learning path references rust-unsafe