[keys-i]

Closes #1.

What changed

• adds one responsive Terminal.Gui dashboard for every adjacent script help page
  under bin/, dev/, and sys/;
• adds keyboard and mouse navigation, filtering, Markdown help, a guided options
  wizard, preview and confirmation gates, live output, progress, and run history;
• adds concise help pages for the Linux, macOS, and Windows cleaners;
• adds a read-only three-OS test workflow and temporary-use instructions.

Why

The cleaners have different platforms and flags, so remembering and running
them directly is error-prone. One shared C# interface keeps their native
implementations authoritative while presenting the same safe workflow on each
operating system.

Checks

• dotnet build dev/tui/Scripts.Tui.csproj --configuration Release --no-restore
• dotnet run --project dev/tui/Scripts.Tui.csproj --configuration Release --no-build -- --self-test
• sys/linux/clean --self-test
• sys/macos/clean --self-test
• PSScriptAnalyzer with zero errors
• workflow YAML parse and git diff --check