fix: stop recommending C# 10+ features that Unity 6 cannot compile

The C# conventions rule told the AI to use file-scoped namespaces and
primary constructors. Unity 6 compiles C# 9, so both fail to build.
Replace them with an explicit C# 9 constraint (#30).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MpEiWjv38TrdFcgqU8ER5L

Author: TMHSDigital

rules/csharp-unity-conventions.mdc

@@ -20,5 +20,5 @@ standards-version: 1.10.0
 - Prefer async/await with Awaitable (Unity 6+) over IEnumerator coroutines for new code
 - An Awaitable instance must never be awaited more than once (it is pooled)
 - Use `FindFirstObjectByType<T>()` instead of the deprecated `FindObjectOfType<T>()`
-- Use file-scoped namespaces to reduce nesting
-- Prefer primary constructors for simple data types
+- Unity 6 compiles C# 9: use block-scoped namespaces (`namespace MyGame { ... }`)
+- Do not use C# 10+ features: file-scoped namespaces, primary constructors on classes or structs, `required` members, raw string literals, collection expressions