refactor: share common files between images; trim fedora-remote for a VM

- Move Kanidm/flatpak/finalize.d/sshd files to images/shared/{files,repos}
  and have build.sh overlay them for every image.
- fedora-remote: aggressive hardware trim (no firmware, no kernel-modules-extra,
  no non-QEMU guest agents), keep + enable qemu-guest-agent (Proxmox).
- Fixes the missing package-list (release step) for fedora-remote.

images/shared/files/flatpak-user-firstboot.service

@@ -0,0 +1,14 @@
+[Unit]
+Description=Install per-user Flatpak applications on first login
+Documentation=https://docs.flatpak.org/en/latest/flatpak-command-reference.html
+ConditionPathExists=!%h/.config/flatpak-user-firstboot.done
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStart=/usr/libexec/flatpak-user-firstboot
+
+[Install]
+WantedBy=default.target