fedora-intune: fix boot issues, add Himmelblau variant

Fixes found by building and inspecting the image:

- Select the Intune authselect profile with-altfiles. Without it system
  users in /usr/lib/passwd (gdm, pipewire, mdatp) do not resolve and the
  login screen never starts. The build now fails if gdm does not resolve.
- Ship /opt as a plain directory (opt-usrlocal: root). Defender installs
  to /opt/microsoft/mdatp and keeps state in /var/opt/microsoft/mdatp;
  with the default /opt -> /var/opt symlink those were the same
  read-only directory. rpm-ostree still relocates /opt content to
  /usr/lib/opt, so finalize.d/60-opt-links links it back (only for images
  where /opt is a directory; fedora-cosmic/remote are unaffected).
- Run Defender's %post on first boot (mdatp-firstboot.service): in a
  compose /var is read-only and the machine GUID/state must not be
  shared. mdatp.service waits for it.

New image fedora-intune-himmelblau: the same workstation with Entra ID
sign-in through Himmelblau (Community Nightly packages), which joins the
device at the first sign-in and enrolls it in Intune (apply_policy)
instead of the Microsoft Intune app. Workstation, Edge and Defender
manifests move to images/shared so both images use them; build.sh copies
images/shared/*.yaml and CI builds both images in one matrix job.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

images/shared/files/mdatp-firstboot.service

@@ -0,0 +1,15 @@
+[Unit]
+Description=Finish the Microsoft Defender for Endpoint install on first boot
+Documentation=https://learn.microsoft.com/en-us/defender-endpoint/microsoft-defender-endpoint-linux
+After=network-online.target
+Wants=network-online.target
+ConditionPathExists=!/var/lib/mdatp-firstboot.done
+
+[Service]
+Type=oneshot
+ExecStart=/usr/libexec/mdatp-firstboot
+RemainAfterExit=yes
+TimeoutStartSec=15min
+
+[Install]
+WantedBy=multi-user.target