fix: strip <think> blocks from final response to users

Fixes #149

The _strip_think_blocks() method existed but was not applied to the
final_response in the normal completion path. This caused <think>...</think>
XML tags to leak into user-facing responses on all platforms (CLI, Telegram,
Discord, Slack, WhatsApp).

Changes:
- Strip think blocks from final_response before returning in normal path (line ~2600)
- Strip think blocks from fallback content when salvaging from prior tool_calls turn

Notes:
- The raw content with think blocks is preserved in messages[] for trajectory
  export - this only affects the user-facing final_response
- The _has_content_after_think_block() check still uses raw content before
  stripping, which is correct for detecting think-only responses

run_agent.py

@@ -2567,7 +2567,8 @@ class AIAgent:
                                             tool_names.append(fn.get("name", "unknown"))
                                         msg["content"] = f"Calling the {', '.join(tool_names)} tool{'s' if len(tool_names) > 1 else ''}..."
                                         break
-                                final_response = fallback
+                                # Strip <think> blocks from fallback content for user display
+                                final_response = self._strip_think_blocks(fallback).strip()
                                 break
                             
                             # No fallback -- append the empty message as-is
@@ -2596,6 +2597,9 @@ class AIAgent:
                     if hasattr(self, '_empty_content_retries'):
                         self._empty_content_retries = 0
                     
+                    # Strip <think> blocks from user-facing response (keep raw in messages for trajectory)
+                    final_response = self._strip_think_blocks(final_response).strip()
+                    
                     final_msg = self._build_assistant_message(assistant_message, finish_reason)
                     
                     messages.append(final_msg)