dsh-living-plan: Maintain a versionable living plan for each DSH session

dsh-living-plan is a DSH workflow plugin (v0.3.1, MIT) released by doctorllll. It maintains a versioned `plan.md` under `$DSH_HOME/living-plan` for each session by injecting `living-plan` messages into the conversation and republishing them after compaction, with the original user speech replaced by a summary. It provides `plan_read/plan_write`, a "Plan" panel in the Web sidebar, and step reminders for things like changing one's mind or progress. It must be installed on DSH 0.2.2-rc.1 using `dsh plugin add github:doctorllll/dsh-living-plan#v0.3.1`. It only takes effect for new sessions and can be fine-tuned via `cordis.patch.yml`.