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)

internal/cli/freshen_test.go

@@ -110,7 +110,10 @@ func TestFreshenRecordEntryMtimePresence(t *testing.T) {
 
 	const relPath = "file1.txt"
 
-	mtime := time.Unix(1_700_000_000, 0)
+	// The scanned file's mtime is the Unix epoch. If recordEntry ever misreads
+	// an absent manifest mtime as the epoch, the "absent" case below would
+	// compare equal to this and be classified unchanged, so the test fails.
+	mtime := time.Unix(0, 0)
 	info := stubFileInfo{size: 8, mtime: mtime}
 
 	for _, tc := range []struct {