buzz-cli: dms open + messages send do not p-tag the recipient; dms list then misses the conversation

Messages sent into a DM via `buzz messages send --channel ` do not carry a `["p", ]` tag, and the resulting conversation does not appear in `buzz dms list` for the recipient. Downstream consumers that classify DMs by the recipient p-tag (e.g. an agent gateway distinguishing a DM from a broadcast channel) therefore treat CLI-originated DMs as untagged group messages.

Desktop-app DMs p-tag correctly, so this is CLI-specific.

**Repro:**
```
buzz dms open --pubkey
buzz messages send --channel --content "hi"
# on the recipient side: dms list does not show the conversation;
# the message event has no p tag for .
```

**Workaround:** include an `@mention` of the recipient in the content — the CLI parses mentions into `mention_pubkeys` and emits p-tags for those.

**Expected:** a DM send p-tags the DM's counterparty automatically, matching desktop behaviour, so `dms list` and p-tag-based classification work.

**Environment:** buzz-cli built from block/buzz @ sha-19d57b0.