fix: stop terminal border flashing with steady cursor and TUI spinner widget

Cherry-picked and improved from PR #470 (fixes #464).

Problem: On Ubuntu 24.04 with ghostty + tmux, the prompt input box
border lines flash due to cursor blink and raw spinner terminal writes
conflicting with prompt_toolkit's rendering.

Changes:
- cli.py: Add CursorShape.BLOCK to Application() to disable cursor blink
- cli.py: Add thinking_callback + spinner_widget in TUI layout so
  thinking status displays as a proper prompt_toolkit widget instead of
  raw terminal writes that conflict with the TUI renderer
- run_agent.py: Add thinking_callback parameter to AIAgent; when set,
  uses the callback instead of KawaiiSpinner for thinking display

What was NOT changed (preserving existing behavior):
- agent/display.py: Untouched. KawaiiSpinner _write() stdout capture,
  _animate() logic, and 0.12s frame interval all preserved. This
  protects subagent stdout redirection and keeps smooth animations
  for non-CLI contexts (gateway, batch runner).
- Original emoji spinner types (brain/sparkle/pulse/moon/star) preserved
  for all non-CLI contexts.

Fixes from original PR #470:
- CursorShape.STEADY_BLOCK -> CursorShape.BLOCK (STEADY_BLOCK doesn't
  exist in prompt_toolkit 3.0.52)
- Removed duplicate self._spinner_text = '' line
- Removed redundant nested if-checks

Tested: 2706 tests pass, interactive CLI verified via tmux.

cli.py

@@ -45,6 +45,11 @@ from prompt_toolkit.widgets import TextArea
 from prompt_toolkit.key_binding import KeyBindings
 from prompt_toolkit import print_formatted_text as _pt_print
 from prompt_toolkit.formatted_text import ANSI as _PT_ANSI
+try:
+    from prompt_toolkit.cursor_shapes import CursorShape
+    _STEADY_CURSOR = CursorShape.BLOCK  # Non-blinking block cursor
+except (ImportError, AttributeError):
+    _STEADY_CURSOR = None
 import threading
 import queue
 
@@ -1187,6 +1192,7 @@ class HermesCLI:
         # History file for persistent input recall across sessions
         self._history_file = Path.home() / ".hermes_history"
         self._last_invalidate: float = 0.0  # throttle UI repaints
+        self._spinner_text: str = ""  # thinking spinner text for TUI
 
     def _invalidate(self, min_interval: float = 0.25) -> None:
         """Throttled UI repaint — prevents terminal blinking on slow/SSH connections."""
@@ -1250,6 +1256,11 @@ class HermesCLI:
 
         return changed
 
+    def _on_thinking(self, text: str) -> None:
+        """Called by agent when thinking starts/stops. Updates TUI spinner."""
+        self._spinner_text = text or ""
+        self._invalidate()
+
     def _ensure_runtime_credentials(self) -> bool:
         """
         Ensure runtime credentials are resolved before agent use.
@@ -1388,6 +1399,7 @@ class HermesCLI:
                 clarify_callback=self._clarify_callback,
                 honcho_session_key=self.session_id,
                 fallback_model=self._fallback_model,
+                thinking_callback=self._on_thinking,
             )
             # Apply any pending title now that the session exists in the DB
             if self._pending_title and self._session_db:
@@ -3666,6 +3678,20 @@ class HermesCLI:
             # right up against the top rule of the input area
             return 1 if cli_ref._agent_running else 0
 
+        def get_spinner_text():
+            txt = cli_ref._spinner_text
+            if not txt:
+                return []
+            return [('class:hint', f'  {txt}')]
+
+        def get_spinner_height():
+            return 1 if cli_ref._spinner_text else 0
+
+        spinner_widget = Window(
+            content=FormattedTextControl(get_spinner_text),
+            height=get_spinner_height,
+        )
+
         spacer = Window(
             content=FormattedTextControl(get_hint_text),
             height=get_hint_height,
@@ -3848,6 +3874,7 @@ class HermesCLI:
                 sudo_widget,
                 approval_widget,
                 clarify_widget,
+                spinner_widget,
                 spacer,
                 input_rule_top,
                 image_bar,
@@ -3902,6 +3929,7 @@ class HermesCLI:
             style=style,
             full_screen=False,
             mouse_support=False,
+            **({'cursor': _STEADY_CURSOR} if _STEADY_CURSOR is not None else {}),
         )
         self._app = app  # Store reference for clarify_callback
         
@@ -3970,6 +3998,7 @@ class HermesCLI:
                         self.chat(user_input, images=submit_images or None)
                     finally:
                         self._agent_running = False
+                        self._spinner_text = ""
                         app.invalidate()  # Refresh status line
                     
                 except Exception as e: