Do we really need /mnt permissions?

Hi,
OpenCode requires these permissions:

```
- --filesystem=/media
- --filesystem=/mnt
- --filesystem=/run/media
```

I removed these permissions on my end, and everything works as expected, even with projects running from a USB stick.
I guess that if we can remove them, it would help clean up some scary nonsense on Flathub. Like this one:

檢查應用程式針對 /media、/mnt 和 /run/media 的 Flatpak 權限宣告，然後驗證 USB 儲存裝置上的專案在沒有這些權限的情況下是否仍能正常運作。完成的標準是移除不必要的權限，同時這些專案情境繼續正常運作；在 issue 討論串中確認任何剩餘的邊界情況。

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