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/fedora-remote/custom.yaml

@@ -20,6 +20,9 @@ packages:
   - foot
   - flatpak
 
+  # Proxmox guest integration.
+  - qemu-guest-agent
+
   # Kanidm Unix authentication (kanidm-unixd-clients pulls kanidm-clients).
   - kanidm-unixd-clients