Discover user-level Copilot instructions in ~/.copilot (#317968)

* Initial plan

* Add home copilot instructions discovery

Co-authored-by: aeschli <6461412+aeschli@users.noreply.github.com>

* update

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: aeschli <6461412+aeschli@users.noreply.github.com>
Co-authored-by: Martin Aeschlimann <martinae@microsoft.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>