chore: Add agentic coding tooling

.crosslink/rules/csharp.md

@@ -0,0 +1,51 @@
+### C# Best Practices
+
+#### Code Style
+- Follow .NET naming conventions (PascalCase for public, camelCase for private)
+- Use `var` when type is obvious from right side
+- Use expression-bodied members for simple methods
+- Enable nullable reference types
+
+```csharp
+// GOOD: Modern C# style
+public class UserService
+{
+    private readonly IUserRepository _repository;
+
+    public UserService(IUserRepository repository)
+        => _repository = repository;
+
+    public async Task<User?> GetUserAsync(string id)
+        => await _repository.FindByIdAsync(id);
+}
+```
+
+#### Error Handling
+- Use specific exception types
+- Never catch and swallow exceptions silently
+- Use `try-finally` or `using` for cleanup
+
+```csharp
+// GOOD: Proper async error handling
+public async Task<Result<User>> GetUserAsync(string id)
+{
+    try
+    {
+        var user = await _repository.FindByIdAsync(id);
+        return user is null
+            ? Result<User>.NotFound()
+            : Result<User>.Ok(user);
+    }
+    catch (DbException ex)
+    {
+        _logger.LogError(ex, "Database error fetching user {Id}", id);
+        throw;
+    }
+}
+```
+
+#### Security
+- Use parameterized queries (never string interpolation for SQL)
+- Validate all input with data annotations or FluentValidation
+- Use ASP.NET's built-in anti-forgery tokens
+- Store secrets in Azure Key Vault or similar