chore: import upstream snapshot with attribution

rules/csharp/coding-style.md

@@ -0,0 +1,72 @@
+---
+paths:
+  - "**/*.cs"
+  - "**/*.csx"
+---
+# C# Coding Style
+
+> This file extends [common/coding-style.md](../common/coding-style.md) with C#-specific content.
+
+## Standards
+
+- Follow current .NET conventions and enable nullable reference types
+- Prefer explicit access modifiers on public and internal APIs
+- Keep files aligned with the primary type they define
+
+## Types and Models
+
+- Prefer `record` or `record struct` for immutable value-like models
+- Use `class` for entities or types with identity and lifecycle
+- Use `interface` for service boundaries and abstractions
+- Avoid `dynamic` in application code; prefer generics or explicit models
+
+```csharp
+public sealed record UserDto(Guid Id, string Email);
+
+public interface IUserRepository
+{
+    Task<UserDto?> FindByIdAsync(Guid id, CancellationToken cancellationToken);
+}
+```
+
+## Immutability
+
+- Prefer `init` setters, constructor parameters, and immutable collections for shared state
+- Do not mutate input models in-place when producing updated state
+
+```csharp
+public sealed record UserProfile(string Name, string Email);
+
+public static UserProfile Rename(UserProfile profile, string name) =>
+    profile with { Name = name };
+```
+
+## Async and Error Handling
+
+- Prefer `async`/`await` over blocking calls like `.Result` or `.Wait()`
+- Pass `CancellationToken` through public async APIs
+- Throw specific exceptions and log with structured properties
+
+```csharp
+public async Task<Order> LoadOrderAsync(
+    Guid orderId,
+    CancellationToken cancellationToken)
+{
+    try
+    {
+        return await repository.FindAsync(orderId, cancellationToken)
+            ?? throw new InvalidOperationException($"Order {orderId} was not found.");
+    }
+    catch (Exception ex)
+    {
+        logger.LogError(ex, "Failed to load order {OrderId}", orderId);
+        throw;
+    }
+}
+```
+
+## Formatting
+
+- Use `dotnet format` for formatting and analyzer fixes
+- Keep `using` directives organized and remove unused imports
+- Prefer expression-bodied members only when they stay readable