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.

run_agent.py

@@ -172,6 +172,7 @@ class AIAgent:
         provider_data_collection: str = None,
         session_id: str = None,
         tool_progress_callback: callable = None,
+        thinking_callback: callable = None,
         clarify_callback: callable = None,
         step_callback: callable = None,
         max_tokens: int = None,
@@ -256,6 +257,7 @@ class AIAgent:
             self.api_mode = "chat_completions"
 
         self.tool_progress_callback = tool_progress_callback
+        self.thinking_callback = thinking_callback
         self.clarify_callback = clarify_callback
         self.step_callback = step_callback
         self._last_reported_tool = None  # Track for "new tool" mode
@@ -3325,9 +3327,13 @@ class AIAgent:
                 # Animated thinking spinner in quiet mode
                 face = random.choice(KawaiiSpinner.KAWAII_THINKING)
                 verb = random.choice(KawaiiSpinner.THINKING_VERBS)
-                spinner_type = random.choice(['brain', 'sparkle', 'pulse', 'moon', 'star'])
-                thinking_spinner = KawaiiSpinner(f"{face} {verb}...", spinner_type=spinner_type)
-                thinking_spinner.start()
+                if self.thinking_callback:
+                    # CLI TUI mode: use prompt_toolkit widget instead of raw spinner
+                    self.thinking_callback(f"{face} {verb}...")
+                else:
+                    spinner_type = random.choice(['brain', 'sparkle', 'pulse', 'moon', 'star'])
+                    thinking_spinner = KawaiiSpinner(f"{face} {verb}...", spinner_type=spinner_type)
+                    thinking_spinner.start()
             
             # Log request details if verbose
             if self.verbose_logging:
@@ -3364,6 +3370,8 @@ class AIAgent:
                     if thinking_spinner:
                         thinking_spinner.stop("")
                         thinking_spinner = None
+                    if self.thinking_callback:
+                        self.thinking_callback("")
                     
                     if not self.quiet_mode:
                         print(f"{self.log_prefix}⏱️  API call completed in {api_duration:.2f}s")
@@ -3404,6 +3412,8 @@ class AIAgent:
                         if thinking_spinner:
                             thinking_spinner.stop(f"(´;ω;`) oops, retrying...")
                             thinking_spinner = None
+                        if self.thinking_callback:
+                            self.thinking_callback("")
                         
                         # This is often rate limiting or provider returning malformed response
                         retry_count += 1
@@ -3573,6 +3583,8 @@ class AIAgent:
                     if thinking_spinner:
                         thinking_spinner.stop("")
                         thinking_spinner = None
+                    if self.thinking_callback:
+                        self.thinking_callback("")
                     api_elapsed = time.time() - api_start_time
                     print(f"{self.log_prefix}⚡ Interrupted during API call.")
                     self._persist_session(messages, conversation_history)
@@ -3585,6 +3597,8 @@ class AIAgent:
                     if thinking_spinner:
                         thinking_spinner.stop(f"(╥_╥) error, retrying...")
                         thinking_spinner = None
+                    if self.thinking_callback:
+                        self.thinking_callback("")
 
                     status_code = getattr(api_error, "status_code", None)
                     if (