[Feature] Implement SSH-slurm / local-slurm integration-test in CI workflow

[aldbr]

User Story

As a contributor for the Slurm scheduler + SSH/Local transports,
I want a containerised single-node Slurm cluster reachable both over SSH and by local exec, in CI,
So that the SSHTransport × Slurm and LocalTransport × Slurm combinations are validated end-to-end.

Feature Description

Stand up the ssh-slurm and local-slurm stacks (IC-ADR-002) — they share one image and
differ only in how the tests reach the batch node:

• ssh-slurm: connect over port 22 (exec_in_container: false);
• local-slurm: docker compose exec pytest inside the container (exec_in_container: true,
  manifest stack: ssh-slurm).

Harmonize onto the scaffold from # per the checklist.

Definition of Done

• Cluster starts cleanly; sinfo shows the compute node idle; healthcheck passes
• Test jobs complete and produce output; CI workflow passes on GH Actions
• sshd variant: key-based access works; ssh-slurm reachable over port 22
• Harmonized onto the scaffold: docker compose, EL9 + pinned Slurm RPM under _images/slurm/,
  stacks/ssh-slurm/ layout, ssh-slurm + local-slurm stacks.yml entries, bespoke
  workflow removed
• SSH creds ephemeral: ssh-keygen in a CI step → authorized_keys, known-hosts via
  ssh-keyscan

Alternatives Considered

Related Issues

Additional Context