chore: import upstream snapshot with attribution

rules/csharp/patterns.md

@@ -0,0 +1,50 @@
+---
+paths:
+  - "**/*.cs"
+  - "**/*.csx"
+---
+# C# Patterns
+
+> This file extends [common/patterns.md](../common/patterns.md) with C#-specific content.
+
+## API Response Pattern
+
+```csharp
+public sealed record ApiResponse<T>(
+    bool Success,
+    T? Data = default,
+    string? Error = null,
+    object? Meta = null);
+```
+
+## Repository Pattern
+
+```csharp
+public interface IRepository<T>
+{
+    Task<IReadOnlyList<T>> FindAllAsync(CancellationToken cancellationToken);
+    Task<T?> FindByIdAsync(Guid id, CancellationToken cancellationToken);
+    Task<T> CreateAsync(T entity, CancellationToken cancellationToken);
+    Task<T> UpdateAsync(T entity, CancellationToken cancellationToken);
+    Task DeleteAsync(Guid id, CancellationToken cancellationToken);
+}
+```
+
+## Options Pattern
+
+Use strongly typed options for config instead of reading raw strings throughout the codebase.
+
+```csharp
+public sealed class PaymentsOptions
+{
+    public const string SectionName = "Payments";
+    public required string BaseUrl { get; init; }
+    public required string ApiKeySecretName { get; init; }
+}
+```
+
+## Dependency Injection
+
+- Depend on interfaces at service boundaries
+- Keep constructors focused; if a service needs too many dependencies, split responsibilities
+- Register lifetimes intentionally: singleton for stateless/shared services, scoped for request data, transient for lightweight pure workers