dsh-essential: Add recoverable conversation deletion to DeepSeek Harness

dsh-essential is an open-source plugin for the DeepSeek Harness Web profile, designed to solve the issue where DSH lacks conversation deletion capabilities. It injects a "Delete conversation" action into the Session row menu and registers a `/delete-conversation` slash command that consumes zero tokens and does not call the model. The core design separates the "intent" from the "execution" of deletion: after confirmation, the Session is immediately hidden via a persistent archive projection, without moving the logs being written by the running process; the actual JSONL directory move is deferred until the next cold start and index building, moving atomically into the trash at `<stateDir>/trash/<session-life`.