docs(rules): give each C# rule statement one authoritative file

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

shared/rules/csharp/references/coding-style.md

@@ -7,7 +7,4 @@ Apply these rules when editing C# syntax, class structure, and everyday code org
 * Keep classes, methods, and responsibilities focused. Use descriptive names and established domain terms; avoid unexplained abbreviations.
 * Prefer composition over inheritance. Use inheritance only for a real substitutable contract, and seal types when extension is not intended.
 * Avoid unnecessary wrappers, abstractions, reflection, runtime type checks, and global mutable state.
-* Prefer immutable or readonly state when it simplifies ownership and reduces accidental mutation.
 * Keep side effects visible. Make behavior deterministic and easy to test where practical.
-* Add comments or API documentation only for non-obvious decisions, public contracts, or documented project requirements.
-* Remove dead code instead of leaving commented-out implementations.