Add agent conversation runtime foundation

mobile/lib/features/channels/channel_messages_provider.dart

@@ -152,8 +152,7 @@ class ChannelMessagesNotifier extends Notifier<AsyncValue<List<NostrEvent>>> {
         content.contains('member_removed');
   }
 
-  /// Kinds that are metadata rather than displayable content (deletions,
-  /// reactions, edits, legacy pre-migration messages).
+  /// reactions, edits, legacy pre-migration messages, task markers).
   static const _metadataKinds = {
     EventKind.deletion,
     EventKind.reaction,
@@ -161,6 +160,8 @@ class ChannelMessagesNotifier extends Notifier<AsyncValue<List<NostrEvent>>> {
     EventKind.streamMessageEdit,
     EventKind.huddleParticipantJoined,
     EventKind.huddleParticipantLeft,
+    EventKind.agentConversationCompat,
+    EventKind.agentConversation,
   };
 
   /// Minimum displayable messages we want after the initial history load.

mobile/lib/shared/relay/nostr_models.dart

@@ -22,7 +22,10 @@ abstract final class EventKind {
   static const dmVisibility = 30622;
   static const streamMessageV2 = 40002;
   static const streamMessageEdit = 40003;
+  static const streamMessagePinned = 40004;
   static const streamMessageDiff = 40008;
+  static const agentConversation = 40010;
+  static const agentConversationCompat = streamMessagePinned;
   static const systemMessage = 40099;
   static const forumPost = 45001;
   static const forumComment = 45003;
@@ -48,7 +51,9 @@ abstract final class EventKind {
     ...channelMessageEventKinds,
     40001, // legacy pre-migration stream messages
     streamMessageEdit, // 40003
+    agentConversationCompat, // 40004 — staging-compatible task marker
     streamMessageDiff, // 40008
+    agentConversation, // 40010 — task marker
     systemMessage, // 40099
     huddleStarted, // 48100 — visible huddle session row
     huddleParticipantJoined, // 48101 — huddle lifecycle metadata