Drop the wrong go.sum reason for not keeping the module cache

The key point under the canonical Go Dockerfile said a kept module
cache would skip checking modules against go.sum. Go checks every
module it takes from the module cache against go.sum, so a kept
module cache skips only the download. The sentence now says that.

Model: opus-5-5

prompts/REPO_POLICIES.md

@@ -280,9 +280,9 @@ 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 and checking
+      run, and the module cache would skip downloading the modules. Go's build
-      them against `go.sum`. Go's build cache also holds test results, which is
+      cache also holds test results, which is why both `go test` runs carry
-      why both `go test` runs carry `-count=1`.
+      `-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: