Mark checkpoints for the DSH exploration workflow

`@dpskh/tool-checkpoint` is a plugin for DeepSeek Harness designed to record the starting point of exploration in multi-tool call scenarios. It resolves the issue of context noise caused by intermediate steps by persisting checkpoint markers. It does not fold content itself but is used in conjunction with `@dpskh/tool-rewind`. The plugin provides a `ctx.checkpoint` service and a checkpoint tool callable by the model, storing markers in local storage and maintaining stable prefixes to optimize KV cache usage. When used, both this plugin and tool-rewind must be mounted: the model first invokes checkpoint to record the starting point, and after completing the exploration, invokes rewind to collapse subsequent steps. Markers are invisible and not logged, and each marker can only be collapsed once.