fix(packaging): address code review findings

- Upgrade path restarts only fenris-collect.timer, not fenris-collect.service
  (spec §7: "a running oneshot finishes on its old interpreter")
- Remove redundant deb depends override in nfpm.yaml (top-level is sufficient)
- Remove common.sh sourcing — scripts are self-contained to avoid path
  dependency when dpkg/rpm run them from /var/lib/dpkg/info/

Co-authored-by: CommandCodeBot <noreply@commandcode.ai>

packaging/preinst.sh

@@ -1,6 +1,5 @@
 #!/bin/sh
 # preinst — abort if make-install remnants detected (spec §7, §9).
-# Dual marker: /var/lib/fenris/manifest.txt or /etc/systemd/system/fenris-collect.timer.
 set -eu
 
 MARKER1="/var/lib/fenris/manifest.txt"