Introducing the dsh-cross-session Plugin for DeepSeek Harness

`dsh-cross-session` is a community plugin that provides cross-session communication capabilities for DeepSeek Harness (DSH). It allows different Sessions within the same process to discover, control, and collaborate with one another without extra daemons or network overhead. The plugin provides core tools for discovery, inspection, sending instructions, steering, waiting, and cancellation, and must be enabled through specific configuration. It is mainly suitable for scenarios requiring parallel task management and context reuse, but has limitations in cross-process usage and resource consumption.