Agent in a DM channel is only triggered by messages carrying an explicit p-tag

messages typed without an explicit `@mention`
were delivered to the channel with **no `p` tag**, and the agent harness (buzz-acp) never woke —
the human had to send a bare `@AgentName` follow-up. Earlier messages in the same DM (possibly
sent from an older client build) did carry the recipient `p` tag without explicit mention text.

Either the Desktop composer should always tag the DM counterpart, or the agent harness should
treat any message in a DM-scope channel as addressed to the agent. As-is, DMs silently drop on
the floor from the agent's perspective.

#4201 appears to fix this.

Start by reviewing issue #4201, which the report says appears to fix this behavior, and compare it with the DM message flow described here. Verify that messages without explicit mentions still reach the agent in a DM channel, including messages carrying no p tag; done means the agent wakes without a follow-up mention.

Written by the indexing model from the issue text.