Allow to set a token limit for a task

### Describe the feature or problem you'd like to solve

Currently each request consumes a certain predicable amount of premium requests - this will change soon and make it completely unpredictable, a task might accidently eat up thwo whole budget for a month.

### Proposed solution

With the upcoming https://github.blog/news-insights/company-news/github-copilot-is-moving-to-usage-based-billing/ it becomes completely unpredictable how much costs a request possibly consumes.

It would therefore be good to set a limit per session / request. If the limit is approached, copilot should ask for confirmation what to do that could be:

1. Complete as soon as possible with what was already accomplished
2. Continue with an increased budget
3. ....

### Example prompts or workflows

_No response_

### Additional context

_No response_

未指定任何檔案、測試或進入點。先定位 CLI 中的任務和工作階段請求處理，然後釐清 token 限制和確認選項應如何運作。完成的標準是：對請求或工作階段套用該限制，且測試涵蓋提議的預算回應。

由索引模型根據 Issue 內容生成。