refactor: upstream-based multi-image pipeline, add fedora-cosmic

Build fedora-cosmic from the upstream Fedora manifests in
fedora/ostree/ci-test (44 and rawhide) with custom overlays:

- remove firefox from the base system
- add token2-fido-bridge + PC/SC smartcard stack and uhid
- install per-user Flatpaks on first login via a systemd user unit

Replace the legacy builder/changelog scripts with:
- build.sh: rpm-ostree compose image (upstream overlay or standalone)
- release.sh: package changelog published as Gitea releases

CI moves to the job-v2 runner with a build matrix, cosign signing and
release publishing. Drop the legacy asahi-cosmic/shared/base images,
builder.sh, changelog.sh, Containerfile and changelogs/.

images/fedora-cosmic/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