docs: document Directory.Build.props/targets hierarchy in agent instruction files (#4984)

* Initial plan

* docs: add Directory.Build.props/targets hierarchy guidance to agent instruction files

Co-Authored-By: GitHub Copilot <copilot@github.com>

Co-authored-by: jamescrosswell <728212+jamescrosswell@users.noreply.github.com>

* Made the instructions more concise

Clarify MSBuild hierarchical structure and overrides.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jamescrosswell <728212+jamescrosswell@users.noreply.github.com>
Co-authored-by: James Crosswell <jamescrosswell@users.noreply.github.com>

Author: Copilot

AGENTS.md

@@ -7,6 +7,12 @@ Uses the .NET SDK (`dotnet` CLI). Key files:
 - `global.json` — pins .NET SDK and Workload versions
 - `Directory.Build.props` / `Directory.Build.targets` — shared MSBuild properties across all projects
 
+### MSBuild hierarchical structure
+
+MSBuild searches upward from each project file and uses the **nearest** `Directory.Build.props`/`.targets` it finds. Parent files are not automatically included. props/targets files in this repo explicitly `<Import>` parent/root files when appropriate.
+
+`integration-test/Directory.Build.props` / `integration-test/Directory.Build.targets` are intentionally isolated (do not import root).
+
 ## Solution Filters
 
 Use solution filters instead of the full `Sentry.slnx`: