docs: document Directory.Build.props/targets hierarchy in agent instruction files

[Copilot]

#skip-changelog

AI coding tools frequently mishandle MSBuild's hierarchical Directory.Build.props/.targets resolution — the nearest file wins and parent files are not automatically inherited unless explicitly imported.

Changes

• AGENTS.md (symlinked as CLAUDE.md and .github/copilot-instructions.md): expanded the Build System section to explain the "nearest wins" MSBuild lookup rule and lists all subdirectory-level overrides:

File(s)	Purpose
src/Directory.Build.{props,targets}	Source packaging, SourceLink, nullability, AOT settings
test/Directory.Build.{props,targets}	xUnit, NSubstitute, Verify test settings
samples/Directory.Build.{props,targets}	Sample app settings
benchmarks/Directory.Build.props	Benchmark settings
integration-test/Directory.Build.{props,targets}	Intentionally isolated — does not import root

All subdirectory files (except integration-test/) explicitly <Import> the root Directory.Build.props/.targets.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add guidance for Directory.Build.props and Directory.Build.targets to agents.md file</issue_title>
<issue_description>question: Directory.Build.props and Directory.Build.targets in sub-directories?

I noticed quite often that AI coding tools don't get MSBuild's hierarchical structure correctly:

1. first Directory.Build.props/.targets found upwards from the project's location
2. more parent Directory.Build.props/.targets files only if imported explicitly

So I'm wondering, if we should mention the other files, too ... something like:

- `samples/Directory.Build.props` / `samples/Directory.Build.targets` - shared MSBuild properties across all projects in the `samples` directory
- `src/Directory.Build.props` / `src/Directory.Build.targets` - shared MSBuild properties across all projects in the `src` directory
- `test/Directory.Build.props` / `test/Directory.Build.targets` - shared MSBuild properties across all projects in the `test` directory

Originally posted by @Flash0ver in #4974 (comment)
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add guidance for Directory.Build.props and Directory.Build.targets to agents.md file #4979

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[github-actions]

Semver Impact of This PR

⚪ None (no version bump detected)

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[Flash0ver]

question: link to Microsoft docs

Shall we have a markdown link to the MSBuild docs?
https://learn.microsoft.com/visualstudio/msbuild/customize-by-directory?view=visualstudio#directorybuildprops-and-directorybuildtargets

Or would links in AGENTS.md just clutter the context?

[jamescrosswell]

The risk is that it follows the link, which will slow it down and add way more context than we really want... so no, I don't think we want to do that. If it keeps stuffing things up re props/targets files even with these instructions, I think we should look at improving these summary/concise instructions rather than linking to docs that are designed to be read by humans.