Pin CLI error messages by driving their real call sites (closes #87)

errmsg_test.go now calls the function that emits each user-visible CLI
error message and asserts on the full text it returns, instead of
rendering format strings copied from production, so rewording any pinned
message fails the suite. The unknown-command message is driven through
the root command's action.

The signer-mismatch case signs a manifest with a throwaway gpg key and
is skipped where gpg is absent, like the repo's other signing tests.

The freshen mtime-presence test gives the scanned file an epoch mtime,
so it fails if recordEntry reads an absent manifest mtime as the epoch.

Model: opus-4-8 (first implementation); opus-5-5 (completion, this message)

TODO.md

@@ -24,6 +24,9 @@ only thing left of the `chore/align-repo-policies` branch is the list below.
 
 # Completed Steps
 
+- 2026-10-03: pinned the CLI error messages by driving the functions that emit
+  them in `internal/cli/errmsg_test.go`, and made the freshen mtime-presence
+  test distinguish an absent mtime from the epoch (#87)
 - 2026-10-03: `script/cibuild` builds the image with the same command as
   `script/docker`, `--no-cache` included, so the checks in the Dockerfile run on
   every build, also on an unchanged tree (#89)