fix: route /bg spinner through TUI widget to prevent status bar collision (#3643)

Background agent's KawaiiSpinner wrote \r-based animation and stop()
messages through StdoutProxy, colliding with prompt_toolkit's status bar.

Two fixes:
- display.py: use isinstance(out, StdoutProxy) instead of fragile
  hasattr+name check for detecting prompt_toolkit's stdout wrapper
- cli.py: silence bg agent's raw spinner (_print_fn=no-op) and route
  thinking updates through the TUI widget only when no foreground
  agent is active; clear spinner text in finally block with same guard

Closes #2718

Co-authored-by: kshitijk4poor <kshitijk4poor@users.noreply.github.com>

cli.py

@@ -4034,6 +4034,17 @@ class HermesCLI:
                     provider_data_collection=self._provider_data_collection,
                     fallback_model=self._fallback_model,
                 )
+                # Silence raw spinner; route thinking through TUI widget when no foreground agent is active.
+                bg_agent._print_fn = lambda *_a, **_kw: None
+
+                def _bg_thinking(text: str) -> None:
+                    # Concurrent bg tasks may race on _spinner_text; acceptable for best-effort UI.
+                    if not self._agent_running:
+                        self._spinner_text = text
+                        if self._app:
+                            self._app.invalidate()
+
+                bg_agent.thinking_callback = _bg_thinking
 
                 result = bg_agent.run_conversation(
                     user_message=prompt,
@@ -4096,6 +4107,9 @@ class HermesCLI:
                 _cprint(f"  ❌ Background task #{task_num} failed: {e}")
             finally:
                 self._background_tasks.pop(task_id, None)
+                # Clear spinner only if no foreground agent owns it
+                if not self._agent_running:
+                    self._spinner_text = ""
                 if self._app:
                     self._invalidate(min_interval=0)