Write the apps snapshot in config init only on macOS (closes #408)

`config init` wrote an `apps` snapshot of `/Applications` on every
system. That directory exists only on macOS, so on Linux the first
`snapshot create` stopped on it unless `--skip-errors` was given.

`defaultConfigText` takes the operating system name and returns the
file with the `apps` snapshot and its comment only for `darwin`;
`config init` passes `runtime.GOOS`. The macOS file is unchanged byte
for byte.

Model: opus-5-5

internal/cli/config_test.go

@@ -13,14 +13,15 @@ import (
 	"sneak.berlin/go/vaultik/internal/ui"
 )
 
-// TestDefaultConfigTemplateParses ensures the init template is valid YAML
-// that unmarshals into the Config struct with the expected snapshots.
-func TestDefaultConfigTemplateParses(t *testing.T) {
+// TestDefaultConfigParsesOnMacOS ensures the config `config init` writes on
+// macOS is valid YAML that unmarshals into the Config struct with the
+// expected snapshots.
+func TestDefaultConfigParsesOnMacOS(t *testing.T) {
 	t.Parallel()
 
 	var cfg config.Config
 
-	err := yaml.Unmarshal([]byte(defaultConfigTemplate), &cfg)
+	err := yaml.Unmarshal([]byte(defaultConfigText("darwin")), &cfg)
 	if err != nil {
 		t.Fatalf("default config template is not valid YAML: %v", err)
 	}
@@ -58,6 +59,39 @@ func TestDefaultConfigTemplateParses(t *testing.T) {
 	}
 }
 
+// TestDefaultConfigOnLinux checks that the config `config init` writes on
+// Linux loads with its home snapshot and without the apps snapshot, whose
+// /Applications exists only on macOS.
+func TestDefaultConfigOnLinux(t *testing.T) {
+	t.Parallel()
+
+	path := filepath.Join(t.TempDir(), "config.yml")
+
+	err := os.WriteFile(path, []byte(defaultConfigText("linux")), configFileMode)
+	if err != nil {
+		t.Fatalf("write config: %v", err)
+	}
+
+	err = writeConfigSet(ui.NewWithColor(&bytes.Buffer{}, false),
+		path, "storage_url", "file:///mnt/backups")
+	if err != nil {
+		t.Fatalf("config set storage_url: %v", err)
+	}
+
+	cfg, err := config.Load(path)
+	if err != nil {
+		t.Fatalf("config.Load: %v", err)
+	}
+
+	if _, ok := cfg.Snapshots["home"]; !ok {
+		t.Error("expected 'home' snapshot in the Linux default config")
+	}
+
+	if _, ok := cfg.Snapshots["apps"]; ok {
+		t.Error("the Linux default config should have no 'apps' snapshot")
+	}
+}
+
 // testRecipient is a valid age public key.
 const testRecipient = "age1278m9q7dp3chsh2dcy82qk27v047zywyvtxwnj4cvt0z65jw6a7q5dqhfj"
 
@@ -71,7 +105,7 @@ func TestConfigSetRecipientOnFreshConfig(t *testing.T) {
 
 	path := filepath.Join(t.TempDir(), "config.yml")
 
-	err := os.WriteFile(path, []byte(defaultConfigTemplate), configFileMode)
+	err := os.WriteFile(path, []byte(defaultConfigText(macOS)), configFileMode)
 	if err != nil {
 		t.Fatalf("write config: %v", err)
 	}
@@ -220,7 +254,7 @@ func TestConfigSetKeepsFileThatWouldNotLoad(t *testing.T) {
 		{levelReference, "blob_size_limit", "1MB",
 			"blob_size_limit must be at least"},
 		// The type check on a config that does not load yet.
-		{defaultConfigTemplate, "snapshots.apps.paths", "/srv",
+		{defaultConfigText(macOS), "snapshots.apps.paths", "/srv",
 			"into []string"},
 		// The type check on a config that cannot be read here.
 		{configWithUnsetReference, "snapshots.home.paths", "/x",
@@ -380,7 +414,7 @@ func loadAfterConfigSet(t *testing.T, key, value string) *config.Config {
 
 	path := filepath.Join(t.TempDir(), "config.yml")
 
-	err := os.WriteFile(path, []byte(defaultConfigTemplate), configFileMode)
+	err := os.WriteFile(path, []byte(defaultConfigText(macOS)), configFileMode)
 	if err != nil {
 		t.Fatalf("write config: %v", err)
 	}