ci: Add Packit CI with RPM builds and TMT integration tests

ci: Add Packit CI with RPM builds and TMT integration tests

Add Packit-based CI pipeline that builds RPMs via COPR and runs TMT
integration tests on bootc image-mode systems. This follows the
bootc-dev/bootc pattern of per-plan VM isolation using bcvk.

Key changes:
- Dockerfile: Add rpmbuild stage, use RPM overlay for rootfs, run
  provision-derived.sh for VM provisioning (cloud-init, rsync, etc.)
- Justfile: Add package target, test-tmt target with bcvk per-plan VMs,
  longer SSH wait for cloud-init first boot
- .github/workflows/bootc.yaml: Split into unit-tests and integration
  jobs, archive TMT logs with PR number in artifact name
- .packit.yaml: COPR builds + TMT tests for centos-stream-9/10 and
  fedora-43/44 on x86_64/aarch64
- tmt/: FMF test plans and shell-based tests (booted verification,
  privileged ostree tests) translated from Rust integration tests
- hack/: Packit provisioning scripts to convert package-mode VMs to
  image-mode via bootc install to-filesystem

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Xiaofeng Wang <henrywangxf@me.com>