Mobile: DM messages omit recipient p-tags, so agents never receive them #3591

Summary

Messages sent from the mobile client in a DM channel carry no p tags, so agent harnesses never receive them as a turn. The event reaches the relay and shows up in history, but the agent stays silent. The same message sent from Desktop works, because Desktop implicitly p-tags every DM participant.

Root cause

Desktop has an explicit DM rule in desktop/src/features/messages/lib/messageMentionPubkeys.ts:

const candidates =
  channel.channelType === "dm"
    ? [...explicitMentions, ...channel.memberPubkeys, ...channel.participantPubkeys]
    : explicitMentions;

Its doc comment names this exact requirement:

Stream messages notify only explicit mentions. A DM addresses every other participant, so it must carry recipient p tags even when the composer text contains no @mention. Agent harnesses and human notification subscriptions both rely on those tags.

Covered by desktop/src/features/messages/lib/messageMentionPubkeys.test.mjs: "plain DM messages p-tag every recipient except the sender".

Mobile has no equivalent. mobile/lib/features/channels/send_message_provider.dart builds tags as:

final tags = <List<String>>[
  ['h', channelId],
  if (parentEventId != null) ..._buildReplyTags(parentEventId, rootEventId),
  for (final pk in normalizedMentions) ['p', pk],
  ...mediaTags,
];

normalizedMentions comes only from explicitly passed pubkeys or from _resolveMentions(), which scans the text for @(\w+). There is no channelType == 'dm' branch anywhere in that file.

Reproduction

1. Open a DM with a managed agent.
2. Send a plain message from Desktop, e.g. hello -> event has ["h", <channel>] and ["p", <agent pubkey>]; the agent replies.
3. Send the same plain message from mobile -> event has only ["h", <channel>] (plus ["e", ...] when replying in a thread); the agent never wakes.

Observed across 15 consecutive messages in one DM: all 9 events with a p tag produced an agent turn, all 6 without one were silently dropped. Correlation was exact; timing, message length and thread nesting made no difference.

Impact

From mobile, a user cannot talk to an agent in a DM at all unless they know to type a literal @AgentName. There is no error and no visual difference: the message appears delivered.

Suggested fix

Port the messageMentionPubkeys rule into send_message_provider.dart: when the channel is a DM, add the other participants to normalizedMentions before building the tags. Mirror the Desktop unit test on the mobile side.

Workaround

On mobile, type @AgentName in the message body so _resolveMentions() produces the p tag.