dsh-agent-messaging: Cross-Session Messaging, Declarations, and Decision Ledger for DeepSeek Harness

dsh-agent-messaging is an MIT-licensed plugin for DeepSeek Harness, designed to address the lack of a direct communication channel between multi-agent sessions, resource conflicts, and duplicate decision-making. By providing cross-session message passing, claim checking, and a decision ledger, this plugin reduces redundant work, contradictions, and deadlocks across sessions. Core features include: using `peer_send` to send short text messages to designated sessions in steer (interrupt), followup (new turn), or context (non-wake) modes; publishing capability cards via `peer_card` to declare session aliases, roles, owned resources, and groupings; and automatically detecting resource claim conflicts while indicating the holder.