Docs: consolidate instructions and fix prompt frontmatter (#44610)

Title: Docs: consolidate Copilot instructions and prompt metadata

## Summary
- Consolidated AI guidance into a root AGENTS.md and new
`.github/instructions` files, removing older per-folder instructions.
- Scoped instruction files for pipelines, common libraries,
runner/settings UI, prompts, and simplified
`.github/copilot-instructions.md` to point to the sources of truth.
- Fixed prompt frontmatter (YAML markers, quoted fields, headings)
across built-in prompt files.
- Most instructions.md is from https://github.com/github/awesome-copilot

## Testing
- Not run (documentation/instructions-only change)

.github/instructions/common-libraries.instructions.md

@@ -0,0 +1,61 @@
+---
+description: 'Guidelines for shared libraries including logging, IPC, settings, DPI, telemetry, and utilities consumed by multiple modules'
+applyTo: 'src/common/**'
+---
+
+# Common Libraries – Shared Code Guidance
+
+Guidelines for modifying shared code in `src/common/`. Changes here can have wide-reaching impact across the entire PowerToys codebase.
+
+## Scope
+
+- Logging infrastructure (`src/common/logger/`)
+- IPC primitives and named pipe utilities
+- Settings serialization and management
+- DPI awareness and scaling utilities
+- Telemetry helpers
+- General utilities (JSON parsing, string helpers, etc.)
+
+## Guidelines
+
+### API Stability
+
+- Avoid breaking public headers/APIs; if changed, search & update all callers
+- Coordinate ABI-impacting struct/class layout changes; keep binary compatibility
+- When modifying public interfaces, grep the entire codebase for usages
+
+### Performance
+
+- Watch perf in hot paths (hooks, timers, serialization)
+- Avoid avoidable allocations in frequently called code
+- Profile changes that touch performance-sensitive areas
+
+### Dependencies
+
+- Ask before adding third-party deps or changing serialization formats
+- New dependencies must be MIT-licensed or approved by PM team
+- Add any new external packages to `NOTICE.md`
+
+### Logging
+
+- C++ logging uses spdlog (`Logger::info`, `Logger::warn`, `Logger::error`, `Logger::debug`)
+- Initialize with `init_logger()` early in startup
+- Keep hot paths quiet – no logging in tight loops or hooks
+
+## Acceptance Criteria
+
+- No unintended ABI breaks
+- No noisy logs in hot paths
+- New non-obvious symbols briefly commented
+- All callers updated when interfaces change
+
+## Code Style
+
+- **C++**: Follow `.clang-format` in `src/`; use Modern C++ patterns per C++ Core Guidelines
+- **C#**: Follow `src/.editorconfig`; enforce StyleCop.Analyzers
+
+## Validation
+
+- Build: `tools\build\build.cmd` from `src/common/` folder
+- Verify no ABI breaks: grep for changed function/struct names across codebase
+- Check logs: ensure no new logging in performance-critical paths