refactor(rules): split C# guidance by topic

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

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+# C# Coding Style
+
+Apply these rules when editing C# syntax, class structure, and everyday code organization.
+
+* Follow current C# and .NET conventions and the style already used in the project.
+* Prefer direct, readable code over clever control flow, hidden magic, or outdated verbosity.
+* 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.