fix(line): persist inbound media in shared store

docs/channels/line.md

@@ -143,6 +143,9 @@ LINE IDs are case-sensitive. Valid IDs look like:
 - Streaming responses are buffered; LINE receives full chunks with a loading
   animation while the agent works.
 - Media downloads are capped by `channels.line.mediaMaxMb` (default 10).
+- Inbound media is saved under `~/.openclaw/media/inbound/` before it is passed
+  to the agent, matching the shared media store used by other bundled channel
+  plugins.
 
 ## Channel data (rich messages)