Show context widget in prompt composer when context reaches >80%

### Short summary

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### Affected version or release

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### Installation context

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### What happened?

In situations where developers like to manually compact, show the context widget when a compaction event is approaching. This approach will keep prompt composer minimal, but still allow control for power users who manually compact.

### Steps to reproduce

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### Expected behavior

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### Additional context

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首先定位 prompt composer 及其 context widget，然后跟踪应用如何检测即将发生的压缩事件。验证当 context 超过 80% 时 widget 会显示，而低于该阈值时 composer 保持最简状态，同时包括手动压缩流程。

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