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/go.md

@@ -137,7 +137,8 @@ CGO_ENABLED=0 go build -trimpath -ldflags="-s -w" -o bin/app ./cmd/app
 - Keep the exported surface small. Unexported by default.
 - Embed frontend/static assets with `//go:embed` for single-binary distribution — the frontend
   itself follows [`js-typescript.md`](skills-public/code-languages/js-typescript.md).
-- File and function size limits from the general rules apply.
+- File and function size limits from the general rules apply — `funlen` in `golangci-lint`
+  covers functions; put the file limit in a test (see [`coding-dev.md` §4](skills-public/coding-dev.md)).
 
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