Add middleware/feature-name casing guidance and a PR review section to Copilot instructions (#37362)

* Initial plan

* Add middleware/feature-name casing guidance and PR review section

* Apply suggestions from code review

Updates per guardrex review suggestions.

Co-authored-by: Wade Pickett <wpickett@microsoft.com>

* Apply suggestion from @wadepickett

Change to "capitalization" to avoid policheck flag.

* Convert unordered-list bullet markers from - to * in copilot-instructions.md

* Apply suggestions from code review

Added review suggestions from @tdykstra.

Co-authored-by: Wade Pickett <wpickett@microsoft.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Wade Pickett <wpickett@microsoft.com>

Author: Copilot