Refuse to freshen a signed manifest without --sign-key (closes #191)

freshen built the new manifest from its flags alone, so a signed
manifest freshened without --sign-key came out unsigned while the log
said its signature verified. freshen now stops after loading such a
manifest, before scanning, with an error naming the signer's
fingerprint and --sign-key, and leaves the manifest as it was. With
--sign-key it rewrites and signs as before.

The library gains Signer() on the loaded manifest: the signing key's
fingerprint, or nil when the manifest is unsigned, as Checker.Signer
gives it.

Model: opus-5-5

internal/cli/freshen.go

@@ -32,6 +32,10 @@ const (
 // modification time where one is required to carry it forward unchanged.
 var errEntryMissingMtime = errors.New("manifest entry has no mtime")
 
+// errSignKeyNeeded indicates a signed manifest given to freshen with no
+// key to sign the rewritten one.
+var errSignKeyNeeded = errors.New("give --sign-key to keep it signed")
+
 // FreshenStatus contains progress information for the freshen operation.
 type FreshenStatus struct {
 	Phase        string        // "scan" or "hash"
@@ -475,9 +479,10 @@ func runFreshenHash(
 }
 
 // loadExistingEntries loads the manifest and indexes its file entries
-// by path.
+// by path. It refuses a signed manifest when no --sign-key is given, as
+// the rewritten manifest would be unsigned.
 func (mfa *CLIApp) loadExistingEntries(
-	manifestPath string,
+	cmd *cli.Command, manifestPath string,
 ) (map[string]*mfer.MFFilePath, error) {
 	log.Infof("loading manifest from %s", manifestPath)
 
@@ -490,6 +495,10 @@ func (mfa *CLIApp) loadExistingEntries(
 		return nil, fmt.Errorf("load manifest: %w", err)
 	}
 
+	if signer := manifest.Signer(); signer != nil && cmd.String("sign-key") == "" {
+		return nil, fmt.Errorf("manifest is signed by %s: %w", signer, errSignKeyNeeded)
+	}
+
 	existingFiles := manifest.Files()
 	log.Infof("manifest contains %d files", len(existingFiles))
 
@@ -520,7 +529,7 @@ func (mfa *CLIApp) freshenManifestOperation(
 		return err
 	}
 
-	existingByPath, err := mfa.loadExistingEntries(manifestPath)
+	existingByPath, err := mfa.loadExistingEntries(cmd, manifestPath)
 	if err != nil {
 		return err
 	}