dsh-stage-gate: DeepSeek Harness Stage Acceptance Gate Plugin

`dsh-stage-gate` is a stage-gate governance plugin for DeepSeek Harness, designed to transform acceptance checkpoints of multi-stage tasks into model-callable tools, thereby reducing reliance on prompt self-awareness. Maintained by changingwang and released under the MIT license, this plugin is applicable to DeepSeek Harness (DSH) version 0.1.0-rc.5 and above in the Web profile. It registers four core tools in `ctx.tools`: `gate_open` is used to create acceptance gates in the "open" state with an acceptance checklist; `gate_check` is used to check acceptance items, supporting the marking of "met", "unmet", or "n/a" along with evidence; and `gate_list` is used to view items in the current session.