fix(coding-dev): prefer a test over the build script for size limits

A shell check only runs where that script runs. A test travels with the repo,
needs no extra tooling, and fails with a message naming the offending files.
Enforcement is now a preference order: native linter rule > test > build
script as fallback, with a worked example and a note on why a per-compile
build hook usually is not worth the incremental-build cost.

Propagated per language: rust (clippy has no file-length lint -> workspace
root test), js-typescript (ESLint max-lines covers both natively, no test
needed), go/python/csharp (linter for functions, test for files), bash (don't
reimplement in the gate what a test can assert).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

code-languages/csharp-dotnet.md

@@ -80,7 +80,8 @@ dotnet build MySolution.sln -warnaserror
 - Pattern matching and switch expressions over `if`/`else` ladders.
 - `IReadOnlyList<T>` / `IReadOnlyDictionary<K,V>` in public signatures returning collections.
 - `internal` by default; `public` only for genuine API surface.
-- File and function size limits from the general rules apply.
+- File and function size limits from the general rules apply — put the file limit in a test
+  rather than an MSBuild target (see [`coding-dev.md` §4](skills-public/coding-dev.md)).
 - `dotnet format` before committing.
 
 ---