Instructions sent to a running build now stick

When you send the agent a new instruction while a build task is already running, that instruction now stays in front of the task until it finishes. Before, it was shown to the task exactly once - mid-work, between two steps - and on a long build the task could lose track of it and finish without it, so you had to ask again. Now a mid-build message keeps its place for the rest of the run, for background tasks and regular chats alike.