dsh-crosstalk: DSH session lateral message plugin

Jesse-njx/dsh-crosstalk is an MIT-licensed plugin based on DeepSeek Harness, designed to address the issue where multiple DSH sessions on the same machine cannot discover or communicate with each other. It extends the `list_agents` and `send_message` functions, enabling any session to list other real-time sessions on the local machine and send messages to them, achieving peer-to-peer communication within the same machine and user environment without the need for additional daemons. Its core mechanisms involve session discovery via heartbeat files in `~/.dsh/crosstalk` and the use of atomic file writes to implement local message delivery. Messages are tagged as requests from peer agents, rather than user commands, ensuring clear trust boundaries.