chore: add .NET agent skills

### Description

### Summary

Add `dotnet/skills` that apply to our development workflows.

### Remarks

Use `skill-scanner` for security: https://github.com/getsentry/skills/blob/main/skills/skill-scanner/SKILL.md
- add to `agents.toml`
- add to `warden.toml`

### See Also
- https://devblogs.microsoft.com/dotnet/extend-your-coding-agent-with-dotnet-skills/
- https://github.com/dotnet/skills
- https://github.com/getsentry/skills/blob/main/skills/skill-scanner/SKILL.md