refactor(instructions): remove deprecated custom instructions

Remove 11 custom instruction files that have been migrated to Agent Skills:
- javascript-typescript.instructions.md
- vue.instructions.md
- nuxt.instructions.md
- css.instructions.md
- tailwind.instructions.md
- html.instructions.md
- csharp.instructions.md
- dotnet.instructions.md
- markdown.instructions.md
- testing.instructions.md
- commit.instructions.md

These files are now Agent Skills with dynamic loading capabilities.
Universal principles (security, accessibility, best practices) remain
as custom instructions since they apply to all code.

Author: devbyray