Add -RunMode flag, VS detection via vswhere, and multi-install selection

- Add -RunMode parameter (Auto/WDK/NuGet/EWDK/Github) defaulting to Auto
- Auto-detection priority order: NuGet -> EWDK -> WDK
- Github mode: same as NuGet but suppresses opening the HTML report
- Add Get-VsInstallationsWithWdk: uses vswhere.exe from its fixed ProgramFiles
  path to find VS installs with the Microsoft.Windows.DriverKit component
- Add Select-VsInstallation: errors if none found, verbose-logs if one found,
  presents a numbered interactive menu if multiple found
- Replace Initialize-DevShell glob-based blind pick with the new helpers
- Remove GITHUB_REPOSITORY auto-detect branch; replace HTML open guard with
  buildEnv.IsGithubMode
- Update comment-based help with -RunMode parameter docs and examples

Co-authored-by: Copilot <[email protected]>

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