dsh-agent-message: Cross-Session Agent Communication Plugin for DeepSeek Harness

dsh-agent-message is a DeepSeek Harness (DSH) plugin designed to enable communication between independent agent sessions within the same process. It allows sessions to exchange messages and query delivery receipts, supporting features such as listing available target sessions, sending messages (with three modes: followup, steer, and inject), checking delivery status, locating target sessions via the @ symbol, displaying visual message cards, and copying session IDs. Installation requires compatibility with Node.js 24 and DSH 0.1.0-rc.6+, and it is enabled via the `dsh plugin` command. This plugin is only applicable to local, non-archived independent sessions and does not support cross-process or cross-machine communication, nor does it include real sub-agents. Usage restrictions include a maximum of 1 delivery between the same pair of sessions within 60 seconds.