Clarify compilation and testing requirements in copilot-instructions

[stephentoub]

Trying to make it more deterministically build before trying to test and commit.

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[Copilot]

Pull request overview

This PR strengthens the compilation and testing requirements in the Copilot instructions to make the build and test process more deterministic before committing code changes.

Changes:

• Upgraded compilation and testing requirements from "SHOULD" to "MUST"
• Removed the exception that allowed skipping builds for comment-only changes
• Clarified that baseline builds must be completed before making changes and running tests
• Consolidated build and test instructions for Libraries component
• Corrected System.Private.CoreLib rebuild configuration from -rc release to -rc checked
• Removed separate build sections for CoreCLR and Mono components (keeping only test instructions)

Comments suppressed due to low confidence (3)

.github/copilot-instructions.md:101

• The removal of the explicit library build command ./build.sh libs -rc release may cause confusion. After the baseline build completes, developers working on library changes need a way to rebuild just the libraries without running tests. The current instructions only show how to build and test a specific library using dotnet build from within that library's directory, but don't provide a command to rebuild all libraries from the repo root after making changes. Consider retaining the ./build.sh libs -rc release command as a separate option for rebuilding all libraries.

This issue also appears in the following locations of the same file:

• line 113
• line 117

**Build and test a specific library:**
```bash
cd src/libraries/<LibraryName>
dotnet build
dotnet build /t:test ./tests/<TestProject>.csproj

**.github/copilot-instructions.md:113**
* The removal of the separate "Build" section for CoreCLR may cause issues. The baseline build (Step 2) for CoreCLR is `./build.sh clr+libs+host`, but after making code changes to CoreCLR, developers need a way to rebuild just the CoreCLR component without rebuilding libs and host. The original `./build.sh clr` command served this purpose. Without it, developers may waste time rebuilding unnecessary components or may not know the correct incremental build command. Consider retaining the build command for incremental rebuilds after the initial baseline.

Test: cd src/tests && ./build.sh && ./run.sh

**.github/copilot-instructions.md:123**
* The removal of the separate "Build" section for Mono may cause issues. The baseline build (Step 2) for Mono is `./build.sh mono+libs`, but after making code changes to Mono, developers need a way to rebuild just the Mono component incrementally. Without an explicit build command, developers may not know how to rebuild after making changes but before running tests. Consider retaining the build command for incremental rebuilds after the initial baseline.

Test:

./build.sh clr.host
cd src/tests
./build.sh mono debug /p:LibrariesConfiguration=debug
./run.sh

</details>

[stephentoub]

/ba-g markdown only