Centralize NuGet README metadata

Author: KSemenenko

AGENTS.md

@@ -168,6 +168,7 @@ If no new rule is detected -> do not update the file.
 - Keep public API and naming aligned with package/namespace `ManagedCode.CodexSharpSDK`.
 - Solution/workspace file naming must use `ManagedCode.CodexSharpSDK` prefix for consistency with package identity.
 - Keep package/version metadata centralized in `Directory.Build.props`; avoid duplicating version structure or release metadata blocks in individual `.csproj` files unless a project-specific override is required.
+- Keep shared NuGet package metadata such as `PackageReadmeFile` in global `Directory.Build.props` for packable projects; do not duplicate it in individual `.csproj` files because package presentation must stay consistent across packages.
 - Never hardcode guessed Codex/OpenAI model names in tests, docs, or defaults; verify supported models and active default via Codex CLI first.
 - Before setting or changing any `Model` value, read available models and current default from the local `codex` CLI in the same environment/account and only then update code/tests/docs.
 - Model identifiers in code/tests must come from centralized constants or a shared resolver helper; do not inline model string literals repeatedly.

Directory.Build.props

@@ -16,6 +16,11 @@
 
   </PropertyGroup>
 
+  <PropertyGroup>
+    <RepositoryReadmeFileName>README.md</RepositoryReadmeFileName>
+    <RepositoryReadmeFilePath>$(MSBuildThisFileDirectory)$(RepositoryReadmeFileName)</RepositoryReadmeFilePath>
+  </PropertyGroup>
+
   <!-- NuGet package metadata and versioning -->
   <PropertyGroup>
     <Authors>ManagedCode</Authors>
@@ -24,16 +29,19 @@
     <RepositoryUrl>https://github.com/managedcode/CodexSharpSDK</RepositoryUrl>
     <PackageProjectUrl>https://github.com/managedcode/CodexSharpSDK</PackageProjectUrl>
     <PackageLicenseExpression>MIT</PackageLicenseExpression>
-    <PackageReadmeFile>README.md</PackageReadmeFile>
     <PublishRepositoryUrl>true</PublishRepositoryUrl>
     <EmbedUntrackedSources>true</EmbedUntrackedSources>
     <EnablePackageValidation>true</EnablePackageValidation>
     <Version>0.1.3</Version>
     <PackageVersion>$(Version)</PackageVersion>
   </PropertyGroup>
 
-  <ItemGroup>
-    <None Include="$(MSBuildThisFileDirectory)README.md" Pack="true" PackagePath="/" Visible="false" />
+  <PropertyGroup Condition="'$(IsPackable)' != 'false'">
+    <PackageReadmeFile>$(RepositoryReadmeFileName)</PackageReadmeFile>
+  </PropertyGroup>
+
+  <ItemGroup Condition="'$(IsPackable)' != 'false'">
+    <None Include="$(RepositoryReadmeFilePath)" Pack="true" PackagePath="/" Visible="false" />
   </ItemGroup>
 
   <PropertyGroup Condition="'$(GITHUB_ACTIONS)' == 'true' or '$(CI)' == 'true' or '$(TF_BUILD)' == 'true'">