Add guidance for `Directory.Build.props` and `Directory.Build.targets` to agents.md file

[jamescrosswell]

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)

[linear]

NET-605