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/prerm.sh

@@ -7,7 +7,7 @@ set -eu
 
 case "${1:-}" in
     remove)
-        # Sanctioned disable — close the monitoring period as user_disabled
+        # Sanctioned disable — close monitoring period (spec §7)
         if [ -x /usr/libexec/fenris/fenris-monitor ]; then
             /usr/libexec/fenris/fenris-monitor disable --now 2>/dev/null || true
         fi