feat(yuanbao): wire native text + media delivery into send_message

_send_yuanbao() already supported media_files= and the user-facing
error strings already advertised yuanbao support, but there was no
dispatch branch in _send_to_platform() actually routing to it. Target
yuanbao in send_message previously fell through to
"Direct sending not yet implemented".

- Add yuanbao media-chunk branch (mirrors Signal/Matrix: media on
  final chunk only).
- Add yuanbao elif in the non-media loop.

Salvage of #17411; SKILL.md description change and redundant
sidebars.ts entry dropped, indentation/trailing-whitespace cleaned up.

tools/send_message_tool.py

@@ -556,6 +556,21 @@ async def _send_to_platform(platform, pconfig, chat_id, message, thread_id=None,
             last_result = result
         return last_result
 
+    # --- Yuanbao: native media attachment support via running gateway adapter ---
+    if platform == Platform.YUANBAO and media_files:
+        last_result = None
+        for i, chunk in enumerate(chunks):
+            is_last = (i == len(chunks) - 1)
+            result = await _send_yuanbao(
+                chat_id,
+                chunk,
+                media_files=media_files if is_last else None,
+            )
+            if isinstance(result, dict) and result.get("error"):
+                return result
+            last_result = result
+        return last_result
+
     # --- Non-media platforms ---
     if media_files and not message.strip():
         return {
@@ -599,6 +614,8 @@ async def _send_to_platform(platform, pconfig, chat_id, message, thread_id=None,
             result = await _send_bluebubbles(pconfig.extra, chat_id, chunk)
         elif platform == Platform.QQBOT:
             result = await _send_qqbot(pconfig, chat_id, chunk)
+        elif platform == Platform.YUANBAO:
+            result = await _send_yuanbao(chat_id, chunk)
         else:
             result = {"error": f"Direct sending not yet implemented for {platform.value}"}