chore: remove deprecated instruction files migrated to agent skills

Remove 11 deprecated instruction files that have been migrated to Agent Skills:
- commit.instructions.md → commit-conventions skill
- csharp.instructions.md → csharp-standards skill
- css.instructions.md → css-standards skill
- dotnet.instructions.md → dotnet-development skill
- html.instructions.md → html-standards skill
- javascript-typescript.instructions.md → javascript-typescript-standards skill
- markdown.instructions.md → markdown-standards skill
- nuxt.instructions.md → nuxt-development skill
- tailwind.instructions.md → tailwind-css skill
- testing.instructions.md → testing-tdd skill
- vue.instructions.md → vue-development skill

These files were marked as DEPRECATED and have been fully replaced by their
corresponding Agent Skills in .github/skills/ directory.

Universal instruction files (accessibility, best-practices, security, etc.)
remain as custom instructions as intended.

Author: devbyray