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

@@ -21,7 +21,7 @@ func TestDefaultConfigParsesOnMacOS(t *testing.T) {
 
 	var cfg config.Config
 
-	err := yaml.Unmarshal([]byte(defaultConfigText(macOS)), &cfg)
+	err := yaml.Unmarshal([]byte(defaultConfigText("darwin")), &cfg)
 	if err != nil {
 		t.Fatalf("default config template is not valid YAML: %v", err)
 	}