✨ 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/csharp.instructions.md

@@ -0,0 +1,17 @@
+---
+description: "Use when writing or modifying C# source code. Covers non-obvious defaults not caught by analyzers."
+applyTo: "**/*.cs"
+---
+# C# Conventions
+
+Code style is enforced by `.editorconfig`, `.globalconfig`, and StyleCop analyzers — do not restate their rules.
+
+## Non-Obvious Defaults
+
+- XML doc comments required on public API in `src/` (suppressed in `tests/` and `samples/`).
+- Avoid `null!` (null-forgiving operator) — fix the nullability flow instead of suppressing the warning.
+
+## Patterns
+
+- Prefer **records** for DTOs, value objects, and other immutable data carriers.
+- Use `LoggerMessage`-generated `partial` methods on a `partial class` — never log inline (e.g., `logger.LogInformation(…)`).