dsh-context-window: Replicating Codex's Context Window Management

`dsh-context-window` is a DSH plugin designed to address information loss caused by traditional summarization compression when the context window fills up. It replicates Codex’s context-window swapping mechanism, using context swapping instead of frequent compression and preserving details through notes and handoff checkpoints. Core functionality includes two modes: the `handoff` mode without summarization (recommended, directly writing to a handoff checkpoint) and the `seam-region` mode with traditional compression. The plugin provides `new_context` and `notes` tools for the model to invoke, and generates handoff information containing the objective, tasks, notes, and the last request when swapping context windows. Note that `handoff` mode does not emit a UI-visible compression event, and swapping is only allowed at step boundaries.