feat: pos media sync — incremental Music→USB sync (mp3/mp4) + shared lib/usb-lib.sh

config/system.env

@@ -1,8 +1,8 @@
 # ~/.config/linux_post_install/system.env — shared "system" tool config
 #
-# Loaded by `pos system health` and `pos system backup` via load_system_env()
-# with precedence: already-exported environment > this file > defaults.
-# Commented lines are defaults — uncomment to override.
+# Loaded by `pos system health`, `pos system backup` and `pos media sync` via
+# load_system_env() with precedence: already-exported environment > this file
+# > defaults. Commented lines are defaults — uncomment to override.
 
 # Roots scanned by `pos system backup --service` and by the backup-age check
 # in `pos system health`. Default: /srv $HOME/srv
@@ -19,9 +19,18 @@
 # cron). The copy lands in <root>/backups/ and is still sha256-verified.
 #BACKUP_USB_ROOT=/mnt/usb
 
-# Where `pos system backup` mounts an unmounted stick it found. Default: /media
-#BACKUP_MOUNT_BASE=/media
+# Where `pos system backup` / `pos media sync` mount an unmounted stick they
+# found. Default: /media (BACKUP_MOUNT_BASE is kept as an alias)
+#USB_MOUNT_BASE=/media
 
 # Dir holding by-id symlinks used to cross-check devices that show no TRAN in
-# lsblk. Default: /dev/disk/by-id
-#BACKUP_USB_BYID=/dev/disk/by-id
+# lsblk. Shared by `pos system backup` and `pos media sync`. Default:
+# /dev/disk/by-id (BACKUP_USB_BYID is kept as an alias)
+#USB_BYID=/dev/disk/by-id
+
+# Source folder synced by `pos media sync`. Default: $HOME/Music
+#MEDIA_SYNC_SOURCE=/home/you/Music
+
+# Subfolder on the USB stick that `pos media sync` mirrors the source into.
+# Default: Music
+#MEDIA_SYNC_DEST=Music