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

@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+# Install the per-user Flatpaks listed in /usr/share/flatpak/flatpaks.list on
+# the first login of each user. Run as a systemd --user oneshot unit.
+set -euo pipefail
+
+LIST="/usr/share/flatpak/flatpaks.list"
+MARKER="${HOME}/.config/flatpak-user-firstboot.done"
+
+[[ -f "${LIST}" ]] || exit 0
+
+# Make Flathub available for the current user.
+flatpak remote-add --user --if-not-exists flathub \
+  https://flathub.org/repo/flathub.flatpakrepo
+
+mapfile -t apps < <(
+  sed -e 's/#.*//' -e 's/[[:space:]]//g' "${LIST}" | grep -v '^$' || true
+)
+
+if [[ ${#apps[@]} -gt 0 ]]; then
+  flatpak install --user --noninteractive --assumeyes "${apps[@]}"
+fi
+
+install -dm0755 "$(dirname "${MARKER}")"
+touch "${MARKER}"