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/50-token2-fido-bridge.preset

@@ -0,0 +1,4 @@
+# Enable the PC/SC smartcard daemon and the token2-fido-bridge daemon so that
+# FIDO2 smartcards are exposed to browsers as a virtual USB-HID security key.
+enable pcscd.socket
+enable token2-fido-bridge.service