refactor(rules): split C# guidance by topic

shared/rules/csharp/index.md

@@ -0,0 +1,14 @@
+# C# Rules
+
+For C# or .NET work, open only the topic rules relevant to the task. Follow repository conventions and load applicable shared security rules as well.
+
+| Topic | Codex rule | Claude skill |
+| --- | --- | --- |
+| Language style and code organization | `references/coding-style.md` | `rules-csharp-coding-style` |
+| Types, collections, and public contracts | `references/types-and-collections.md` | `rules-csharp-types-and-collections` |
+| Architecture and API boundaries | `references/architecture-and-apis.md` | `rules-csharp-architecture-and-apis` |
+| Async work and concurrency | `references/async-and-concurrency.md` | `rules-csharp-async-and-concurrency` |
+| Performance | `references/performance.md` | `rules-csharp-performance` |
+| Security | `references/security.md` | `rules-csharp-security` |
+| Validation, errors, and logging | `references/safety-and-errors.md` | `rules-csharp-safety-and-errors` |
+| Tests, dependencies, and analyzers | `references/testing-and-dependencies.md` | `rules-csharp-testing-and-dependencies` |