fix: show current mode for latest assistant message when assistant_mode is missing

Display-time fallback for the most recent assistant message to show the current mode name when assistant_mode field hasn't been stamped yet. This fixes new messages showing generic 'ASSISTANT' instead of the active mode label (e.g., 'NEOAGENT').

Historical messages remain unchanged, preserving their original mode labels or showing generic 'ASSISTANT' for old messages created before mode tracking was added.

Author: Danilo Verde Ribeiro

lua/opencode/ui/session_formatter.lua

@@ -300,7 +300,14 @@ function M._format_message_header(message, msg_idx)
     if mode and mode ~= '' then
       display_name = mode:upper()
     else
-      display_name = 'ASSISTANT'
+      -- For the most recent assistant message, show current_mode if assistant_mode is missing
+      -- This handles new messages that haven't been stamped yet
+      local is_last_message = msg_idx == #state.messages
+      if is_last_message and state.current_mode and state.current_mode ~= '' then
+        display_name = state.current_mode:upper()
+      else
+        display_name = 'ASSISTANT'
+      end
     end
   else
     display_name = role:upper()