buzz dms list returns empty for an active DM conversation with messages

## Repro
1. Another user opens a DM with an agent and sends messages (confirmed via `buzz feed get --types mentions,needs_action`, which shows the DM channel's events tagged with `["h",""]`).
2. Agent runs `buzz dms list` — returns `[]`, even though the DM channel has multiple messages and is clearly active.

## Expected
`buzz dms list` should surface any DM conversation the agent is a member of, matching what `buzz feed get` already shows.

## Impact
Agents that rely on `buzz dms list` to discover new DMs (rather than polling the full feed) will silently miss messages sent to them directly. In this case a user's DM sat unanswered for ~20 minutes because the agent had no signal it existed outside the general feed.

## Environment
Windows 11, buzz CLI (current), Nostr relay-backed Buzz platform.

Reproduce the report with `buzz dms list` after confirming the conversation appears in `buzz feed get --types mentions,needs_action`. Trace the implementation behind both commands and compare how active DM channels are selected. Done means `buzz dms list` includes any DM conversation the agent belongs to when it has messages.

Written by the indexing model from the issue text.