fix: resolve all test failures and Cubic review issues

- Fix unstable-agent-babysitter: add promptAsync to test mock
- Fix claude-code-mcp-loader: isolate tests from user home configs
- Fix npm-dist-tags: encode packageName for scoped packages
- Fix agent-builder: clone source to prevent shared object mutation
- Fix add-plugin-to-opencode-config: handle JSONC with leading comments
- Fix auth-plugins/add-provider-config: error on parse failures
- Fix bun-install: clear timeout on completion
- Fix git-diff-stats: include untracked files in diff summary

src/cli/config-manager/auth-plugins.ts

@@ -35,10 +35,13 @@ export async function addAuthPlugins(config: InstallConfig): Promise<ConfigMerge
     if (format !== "none") {
       const parseResult = parseOpenCodeConfigFileWithError(path)
       if (parseResult.error && !parseResult.config) {
-        existingConfig = {}
-      } else {
-        existingConfig = parseResult.config
+        return {
+          success: false,
+          configPath: path,
+          error: `Failed to parse config file: ${parseResult.error}`,
+        }
       }
+      existingConfig = parseResult.config
     }
 
     const plugins: string[] = existingConfig?.plugin ?? []