✨ Adds project guidelines and conventions for Github Copilot

Introduces detailed instructions for ASP.NET API, C#, Docker, MSBuild,
testing, and library creation. These guidelines aim to standardize
development practices and improve code quality across the project.

.github/instructions/msbuild.instructions.md

@@ -0,0 +1,39 @@
+---
+description: "Use when editing MSBuild files (.csproj, .props, .targets). Covers central package management, artifacts layout, and property inheritance."
+applyTo: "**/*.{csproj,props,targets}"
+---
+# MSBuild Conventions
+
+## Central Package Management
+
+All package versions are declared in `Directory.Packages.props` at the repo root. In `.csproj` files:
+
+```xml
+<!-- Correct -->
+<PackageReference Include="Newtonsoft.Json" />
+
+<!-- Wrong — never specify Version -->
+<PackageReference Include="Newtonsoft.Json" Version="13.0.3" />
+```
+
+To add or update a version, edit `Directory.Packages.props`:
+
+```xml
+<PackageVersion Include="Newtonsoft.Json" Version="13.0.3" />
+```
+
+## Properties Set Centrally (Do Not Override)
+
+These are configured in the root `Directory.Build.props` — do **not** set them in individual projects:
+
+`LangVersion`, `Nullable`, `ImplicitUsings`.
+
+## InternalsVisibleTo
+
+Handled automatically via `<InternalsVisibleToSuffix>` in the root props. Default suffixes are `.Tests` and `Tests`. To add custom visibility:
+
+<parameter name="newString">```xml
+<ItemGroup>
+  <InternalsVisibleTo Include="MyProject.IntegrationTests" />
+</ItemGroup>
+```