File sync sessions count as workspace activity

Coder Desktop file sync sessions communicate with the workspace over SSH, even when both sides of the file sync are idle. I realised this when my workspace hadn't autostopped for over two weeks due to a file sync session I had configured to it on my laptop.

I think it's reasonable to assume that someone using file sync will also have some other connection to their workspace (IDE, SSH) that would bump workspace activity whilst it's actually in use.
One way to fix this might be to modify Mutagen to populate an environment variable over SSH, and then update the Coder agent to not count SSH sessions with that environment variable as authentic activity.

まず、Coder Desktop と Mutagen の file-sync SSH セッションがワークスペースのアクティビティとしてどのようにカウントされるかを追跡し、次に Coder agent が SSH 環境変数をどのように受け取るかを調べます。アイドル状態の file-sync セッションが autostop を妨げなくなり、IDE 接続またはインタラクティブな SSH 接続は引き続きアクティビティとしてカウントされることを確認します。

索引モデルが issue の本文から書いたものです。