M78 step 2. The /disk page surfaced reclaimable space + stopped counts but
said "prune deferred to a later milestone" — this wires the cleanup.
Adds POST /plugins/docker/disk/<host_id>/prune (admin only) that resolves
the host's linked AnsibleTarget → steward:target:<id> scope and fires the
bundled maintenance/docker_prune.yml through the ansible.run_playbook
capability (audited runner; the collection agent stays read-only). Mirrors
the host_agent update route. `target` ∈ {containers, images, system}:
- containers → docker container prune -f
- images → docker image prune -af (prune_all_images)
- system → docker system prune -f (conservative)
The disk() route now resolves each host group's linked target + capability
availability; disk.html renders three confirm-gated buttons per host, shown
only to admins when Ansible is available and the host has a linked target,
otherwise a clear "link a target" / "runner unavailable" hint (rules 24/26/27).
Tests: the prune_target→extra_vars mapping (the -af-only-for-images rule) is
extracted to a pure helper and unit-tested; disk_prune added to the module
smoke; disk.html covered by the existing template-parse test.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CAGR73DUowdVFVvYzLXC5C
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