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/signing_test.go

@@ -73,6 +73,56 @@ func TestGenAndFreshenSignWithKeyFile(t *testing.T) {
 	}
 }
 
+// TestFreshenSignedManifestNeedsSignKey runs freshen on a signed manifest
+// after a file is added. Without --sign-key it must fail, naming the
+// signer and --sign-key, and leave the manifest as it was; with --sign-key
+// it must rewrite the manifest, signed.
+func TestFreshenSignedManifestNeedsSignKey(t *testing.T) {
+	t.Setenv(envSignKeyPassphrase, "")
+
+	secretKey, fingerprint := testSecretKey(t, nil, nil)
+
+	fs := afero.NewOsFs()
+	keyFile := filepath.Join(t.TempDir(), "key.asc")
+	root := t.TempDir()
+	manifestPath := filepath.Join(root, defaultManifestName)
+
+	require.NoError(t, afero.WriteFile(fs, keyFile, secretKey, 0o600))
+	writeTestFile(t, fs, filepath.Join(root, testFileTxt), "hello")
+
+	opts := testOpts([]string{
+		testApp, cmdGenerate, "-q", testFlagSignKey, keyFile,
+		"-o", manifestPath, root,
+	}, fs)
+	require.Equal(t, 0, runCLI(opts), testStderr(t, opts))
+
+	before, err := afero.ReadFile(fs, manifestPath)
+	require.NoError(t, err)
+
+	writeTestFile(t, fs, filepath.Join(root, "added.txt"), "added")
+
+	opts = testOpts([]string{testApp, cmdFreshen, "-q", manifestPath}, fs)
+	assert.Equal(t, 1, runCLI(opts))
+	assert.Contains(t, testStderr(t, opts), fingerprint)
+	assert.Contains(t, testStderr(t, opts), testFlagSignKey)
+
+	after, err := afero.ReadFile(fs, manifestPath)
+	require.NoError(t, err)
+	assert.Equal(t, before, after)
+
+	opts = testOpts([]string{
+		testApp, cmdFreshen, "-q", testFlagSignKey, keyFile, manifestPath,
+	}, fs)
+	require.Equal(t, 0, runCLI(opts), testStderr(t, opts))
+
+	opts = testOpts([]string{
+		testApp, cmdCheck, "-q",
+		"--" + flagRequireSignature, fingerprint, manifestPath,
+	}, fs)
+	require.Equal(t, 0, runCLI(opts), testStderr(t, opts))
+	assert.Len(t, manifestFiles(t, fs, manifestPath), 2)
+}
+
 // TestSignWithProtectedKeyNeedsPassphrase runs gen with a protected key,
 // with MFER_SIGN_KEY_PASSPHRASE empty and no terminal to ask on. gen must
 // fail, naming the variable, and write no manifest.