feat(opencode): add dotnet formatter

[workedbeforepush]

Issue for this PR

Closes #16435

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

• add formatter dotnet-format support for .NET files
• resolve the nearest .NET project or solution and run dotnet format from that directory with file-scoped includes
• document the formatter and add config support for passing formatter args (e.g. to change severity)

How did you verify your code works?

• tested a real C# file edit in a test project
• confirmed OpenCode reformatted the file and logged ["dotnet","format","DotNetFormatter.csproj","--include","Program.cs","--severity","info"]

Screenshots / recordings

No UI changes made.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

If you do not follow this template your PR will be automatically rejected.