fix(filesystem): correct write success message from bytes to characters

len(content) counts Unicode code points, not UTF-8 bytes. For non-ASCII
content such as Chinese or emoji, the reported count would be lower than
the actual bytes written to disk, which is misleading to the agent.

nanobot/agent/tools/filesystem.py

@@ -186,7 +186,7 @@ class WriteFileTool(_FsTool):
             fp = self._resolve(path)
             fp.parent.mkdir(parents=True, exist_ok=True)
             fp.write_text(content, encoding="utf-8")
-            return f"Successfully wrote {len(content)} bytes to {fp}"
+            return f"Successfully wrote {len(content)} characters to {fp}"
         except PermissionError as e:
             return f"Error: {e}"
         except Exception as e: