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/.

.gitignore

@@ -1,10 +1,4 @@
 /*.ociarchive
 /.buildid
+/build/
 /cache/
-/debugdata/
-/fedora-comps/
-/fedora-lorax-templates/
-/iso/
-/logs/
-/repo/
-/tmp/