[Feature]: loop增加压缩上下文管理命令

## Summary
loop增加压缩上下文管理命令：loop的目标、任务模式增加压缩（/compact）的命令，长任务每次提交都把超长的上下文直接提交，导致token消耗巨量，希望在长目标、任务模式里能增加压缩（/compact）的命令，当上下文达到多大时运行压缩（/compact）的命令后再继续。

Start from the loop command-handling path that interprets slash commands and from the context-management flow used by target/task mode. Locate where turns are appended and where long-context state is maintained, then add support for `/compact` and a compression trigger when context grows too large. Validate by running a long-running loop task and confirming `/compact` works and token-heavy context is reduced before the run continues.

Rédigé par le modèle d'indexation à partir du texte de l'issue.