Add the first copilot-instructions.md (#41518)

This pull request introduces concise, area-specific contributor guides
for the PowerToys repository. Each major code area now has its own
instructions file, clarifying scope, coding guidelines, and acceptance
criteria. This helps ensure consistency, reduces onboarding friction,
and sets clear expectations for contributors.
Reference doc:
https://docs.github.com/en/copilot/how-tos/configure-custom-instructions/add-repository-instructions

New contributor guides added:

**General/Top-level guidance**
- Added `.github/copilot-instructions.md` as a concise, top-level guide
for AI-driven changes, including a repo map, build/test workflow, PR
expectations, and quick reference checklists.

**Area-specific instructions**
- Added `src/common/common.instructions.md` for shared libraries,
covering ABI stability, logging, performance, and dependency policies.
- Added `src/runner/runner.instructions.md` for the Runner/tray host,
detailing module management, IPC contract alignment, startup
performance, and elevation/update logic.
- Added `src/settings-ui/settings-ui.instructions.md` for the Settings
UI, with guidance on schema changes, IPC contract sync, UI
responsiveness, and style reuse.

src/settings-ui/settings-ui.instructions.md

@@ -0,0 +1,17 @@
+---
+applyTo: "**/*.cs,**/*.xaml"
+---
+# Settings UI – configuration app guidance
+
+Scope
+- WinUI/WPF UI, communicates with Runner over named pipes; manages persisted settings schema.
+
+Guidelines
+- Don’t break settings schema silently; add migration when shape changes.
+- If IPC/JSON contracts change, align with `src/runner/**` implementation.
+- Keep UI responsive: marshal to UI thread for UI-bound operations.
+- Reuse existing styles/resources; avoid duplicate theme keys.
+- Add/adjust migration or serialization tests when changing persisted settings.
+
+Acceptance
+- Schema integrity preserved, responsive UI, consistent contracts, no style duplication.