Give a reason that holds for not keeping the module cache

Go builds from whatever files a kept module cache holds without
checking them again, so a changed file would go unnoticed; skipping a
download skipped no check and was no reason.

Model: opus-5-5

prompts/REPO_POLICIES.md

@@ -280,9 +280,10 @@ style conventions are in separate documents:
       check builds, and Go reuses one only when all its inputs are unchanged, so
       it saves the compile without skipping a check. Nothing else is kept:
       golangci-lint's own cache would replay stored findings in place of a lint
-      run, and the module cache would skip downloading the modules. Go's build
+      run, and Go builds from whatever files a kept module cache holds without
-      cache also holds test results, which is why both `go test` runs carry
+      checking them again (only `go mod verify` does), so a file changed there
-      `-count=1`.
+      would go unnoticed. Go's build cache also holds test results, which is why
+      both `go test` runs carry `-count=1`.
     - If the project uses `//go:embed` directives that reference build artifacts
       (e.g. a web frontend compiled in a separate stage), the lint phase must
       create placeholder files so the embed directives resolve. Example: