✨ 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/aspnet-api.instructions.md

@@ -0,0 +1,32 @@
+---
+description: "Use when writing API endpoints, routing, middleware, request/response handling, OpenAPI configuration, or HTTP pipeline setup in ASP.NET. Covers Minimal API conventions, error handling, and endpoint organisation."
+---
+# ASP.NET API Conventions
+
+## Routing
+
+- All endpoints live under `/api/v{apiVersion}/{resource}`.
+- Default to `/api/v1` for projects without explicit API versioning.
+
+## Endpoints
+
+- Prefer **Minimal APIs**; use Controllers only when the added structure is justified.
+- Group related endpoints using `MapGroup()` with a shared prefix and tag.
+- Use **typed results** (`TypedResults.Ok()`, `TypedResults.NotFound()`) over untyped `Results` for OpenAPI metadata inference.
+- Return **`ProblemDetails`** (RFC 9457) for all error responses — configure via `AddProblemDetails()`.
+
+## Request Handling
+
+- Bind request data with **`[AsParameters]`** for complex parameter sets.
+- Validate input at the endpoint boundary — use **filters** or **`IValidatableObject`** to fail fast.
+- Use **`CancellationToken`** on all async endpoints.
+
+## Endpoint Organisation
+
+- One file per resource or feature area (e.g., `UserEndpoints.cs`, `OrderEndpoints.cs`).
+- Register via extension methods on `IEndpointRouteBuilder`: `MapUserEndpoints()`.
+- Keep endpoint lambdas thin — delegate business logic to injected services.
+
+## OpenAPI
+
+- Annotate endpoints with `.WithTags()` for grouping in the generated spec.