fix(trigger): defer local triggers until session idle

docs/webui.md

@@ -133,9 +133,11 @@ that webhook/service outside nanobot and have it call the trigger command with
 the final message.
 
 Trigger deliveries use the same workspace as the gateway. They survive gateway
-restarts and are requeued if the process exits before marking a delivery
-complete. This is an at-least-once local queue, so repeated delivery is possible
-after an interrupted process.
+restarts and are requeued if the process exits before the linked turn completes.
+If the linked session is already running a turn, the local trigger waits until
+that session is idle instead of being injected into the active turn. This is an
+at-least-once local queue, so repeated delivery is possible after an interrupted
+process.
 
 For recurring background checks that should stay quiet unless there is something
 useful to report, use the protected heartbeat job by editing `HEARTBEAT.md`