✨ Add Copilot instructions for Splat repository (#1394)

* Initial plan

* Add Copilot instructions for Splat repository

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

* Add comprehensive ReactiveUI C# Style Guide to Copilot instructions

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

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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: glennawatson <[email protected]>

Author: Copilot