fix(agent): remove hardcoded default timezone, ask on-demand instead

Instead of assuming Asia/Saigon or UTC for all users, the model now
asks for timezone when the user mentions times/schedules/reminders.
After first ask, timezone is stored in USER.md for future sessions.

Closes #833 discussion.

internal/agent/loop_history.go

@@ -96,7 +96,7 @@ func (l *Loop) buildMessages(ctx context.Context, history []providers.Message, s
 	// DM only — group chats have permission checks and multiple senders.
 	if hadBootstrap && peerKind == "direct" {
 		if senderName := store.SenderNameFromContext(ctx); senderName != "" {
-			hint := fmt.Sprintf("Known user info (from %s): Name=%q\nDefault timezone: Asia/Saigon (GMT+7). User can correct this.", channelType, senderName)
+			hint := fmt.Sprintf("Known user info (from %s): Name=%q\nTimezone: not yet known. When the user mentions times, schedules, or reminders, ask for their timezone and update USER.md.", channelType, senderName)
 			if extraSystemPrompt != "" {
 				extraSystemPrompt += "\n\n"
 			}