exclude samba msg.sock from system mirror

Sockets are ephemeral IPC endpoints with PID-based names — they get
recreated on every samba restart and can't be backed up meaningfully.
Was causing every nightly rsync to USB to recopy them.

scripts/sync-system.sh

@@ -21,7 +21,7 @@ sed -i 's/^password .*/password REDACTED/' "$REPO/system/msmtprc"
 # rsync of /opt/argento/ picks them up. Gitignored — credential hashes don't
 # belong in version control, but they DO need to ride along on the USB backup.
 install -d -m 700 "$REPO/system/samba-private"
-rsync -a --delete /var/lib/samba/private/ "$REPO/system/samba-private/"
+rsync -a --delete --exclude='msg.sock/' /var/lib/samba/private/ "$REPO/system/samba-private/"
 
 # Sync root crontab
 crontab -l > "$REPO/system/root-crontab" 2>/dev/null || true