Clarify CLI-first workflow and example validation

[PrzemyslawKlys]

Summary

• rewrite the README around the PowerShell-authoring and CLI-runtime workflow
• make the repo example build current binaries, export a portable JSON config, and run the CLI against it
• add PowerShell coverage that exported JSON preserves builtin tokens and that CLI output matches PowerShell output

Validation

• dotnet test Sources/PowerBGInfo.sln
• Invoke-Pester -Path 'Sources/PowerBGInfo.Tests/Cmdlet.Tests.ps1' -Output Detailed
• pwsh -NoLogo -NoProfile -File Examples/Run.BGInfo4.ps1

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