Reorganize files by ownership

[danielchalmers]

Summary

• Move action models/defaults/execution helpers into RadialActions/Actions
• Move hotkey helpers into RadialActions/Hotkeys and WPF helpers into RadialActions/Interop
• Mirror production ownership folders in RadialActions.Tests
• Update AGENTS.md with the new project map and ownership guidance

Validation

• dotnet build RadialActions.sln
• dotnet test RadialActions.sln

Risks

• Low: this is a file reorganization only, with namespaces and code behavior left unchanged.