feat(ansible): playbooks self-describe via "# description:" comment

Playbooks can ship a human description Steward reads and shows when one is
selected. Convention: a `# description: <text>` magic comment (Ansible rejects
unknown play keys, so a comment is the portable place — works for third-party
playbooks too); falls back to the first play's name:. sources
.discover_playbook_description().

Surfaced at the top of the shared _playbook_vars.html partial, which loads on
playbook selection in the host run form, schedules form, and browse run form.
All four bundled playbooks (provision/install/update/docker_prune) now carry a
description line. Unit tests added.

Scribe #900.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

steward/ansible/bundled/maintenance/docker_prune.yml

@@ -1,4 +1,5 @@
 ---
+# description: Reclaim disk on Docker / Swarm nodes by pruning unused images, containers, networks and build cache.
 # Reclaim disk on Docker / Docker Swarm nodes by removing unused data.
 # Safe by default: prunes dangling images, stopped containers, unused networks
 # and build cache. Set extra-vars to widen scope: