fix: update with mirror

[Paulkm2006]

Summary

• fixed an error when updating npm-managed agents with mirror
• improved update log display
• correctly parse command exit status to task status

Verification

• Tests were added or updated for behavior changes, or the reason they are unnecessary is explained.
• Relevant Go, frontend, documentation, and release-compliance checks pass locally.
• UI changes include screenshots or a short recording, or this change has no UI impact.

Change checklist

• No API key, token, private configuration, or sensitive path is present in code, logs, screenshots, fixtures, or issue links.
• No Wails DTO or service changed, or frontend/bindings, frontend/src/backend/wails.ts, and handwritten API types were synchronized.
• No documented user-visible behavior changed, or README.md and README_ZH.md were updated together.
• No distributed dependency or third-party mark was added, or NOTICE was updated.
• Public documentation is in English; AGENTS.md and docs/internal/ remain in Chinese.