fix(tui): stop a stream outlived by /new from leaking into the next conversation

_finish_stream appended the completed reply to self.history - whatever
list that name currently pointed at - not to the conversation the
stream was actually answering. /new reassigns self.history to a fresh
list; a stream still running when that happens finished by silently
appending the old conversation's trailing reply onto the new one, which
then rides along in that new conversation's next /chat/stream history
payload. The conversation_id was already captured by closure for this
exact reason (see the tool-denial path); self.history needed the same
treatment.

nexusos_cli/tui_app.py

@@ -359,14 +359,19 @@ class NexusTUI:
                 if not self.conversation_id:
                     self.conversation_id = str(uuid.uuid4())
                 conversation_id = self.conversation_id
+                # The list object itself, not self.history - /new reassigns
+                # self.history to a fresh list, and a stream that outlives that
+                # must keep appending its reply to the conversation it actually
+                # belongs to, not whatever self.history now points at.
+                history_ref = self.history
                 body: dict[str, Any] = {
                     "message": message,
                     "conversation_id": conversation_id,
-                    "history": list(self.history),
+                    "history": list(history_ref),
                 }
                 if self._model:
                     body["model"] = self._model
-                self.history.append({"role": "user", "content": message})
+                history_ref.append({"role": "user", "content": message})
 
                 async def stream_worker():
                     reply_parts: list[str] = []
@@ -474,19 +479,19 @@ class NexusTUI:
                         if self._stream_cancel == (loop, task):
                             self._stream_cancel = None
                         text = "".join(reply_parts).strip()
-                        self._call_ui(self._finish_stream, text)
+                        self._call_ui(self._finish_stream, text, history_ref)
 
                 threading.Thread(
                     target=lambda: asyncio.run(stream_worker()), daemon=True
                 ).start()
 
-            def _finish_stream(self, text: str) -> None:
+            def _finish_stream(self, text: str, history_ref: list) -> None:
                 log = self.query_one("#log", RichLog)
                 live = self.query_one("#live", Static)
                 try:
                     if text:
                         log.write(format_assistant_line(text))
-                        self.history.append(
+                        history_ref.append(
                             {"role": "assistant", "content": text}
                         )
                 finally: