dsh-code-pipeline: Injecting Stage Sub-Agent Tools into PTC Code Mode Pipelines

`dsh-code-pipeline` is a dynamic pipeline management tool provided as a DSH bundle plugin, designed to address the rigid pipeline model configuration in PTC Code Mode. The plugin dynamically injects sub-agent tools by listening to events and enables runtime model configuration via the settings page without restarting the session. Core features include dynamic model configuration, background execution mode, multi-round review reuse, and on-disk message persistence. It also provides the `pipeline_followup` tool for handling requirement changes during multi-round reviews and supports manual gate approval at the Plan stage. The plugin requires Node.js >= 22.19.0 and dsh >= 0.1.6-alpha.1.