dsh-plan-build-toggle: DeepSeek Harness Persistent Plan/Build Toggle

`dsh-plan-build-toggle` is a DeepSeek Harness plugin that replicates OpenChamber-style mode control. It provides the composer with persistent native `/plan` command control via an always-visible `Plan｜Build` pill and Tab key toggling, supporting bidirectional switching and inline error display. Restart the app after installation. Note that this plugin only works for sessions that have a `plan` projection.