feat(todo): auto-cleanup stale todos + /clear-tasks and /清除任务 commands

[LifetimeVip]

Issue for this PR

N/A - no related issue. Feature requested in community discussion.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Stale todos accumulate in the chat UI and persist across conversations. This PR adds auto-cleanup logic and two slash commands to manage active tasks.

Changes:

• Auto-cleanup: Completed/cancelled todos are filtered out before saving (tool/todo.ts) and on session load (session/todo.ts)
• /clear-tasks and /清除任务: New slash commands to clear all active tasks at once
• i18n (zh.ts): Chinese translations for the new command descriptions

These are safe backend-only improvements — no UI changes, no breaking changes.

How did you verify your code works?

• Ran npm run typecheck — zero errors
• Ran npm test — 62/62 all passing
• Manually verified todos are filtered correctly on reload

Screenshots / recordings

N/A — backend-only change, no UI.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[LifetimeVip]

recheck

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[LifetimeVip]

Hi @adamdotdevin, this is my first PR to this repo. The CI workflows are pending approval — could you please approve and run them? Thanks!