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/templates/ansible/_playbook_vars.html

@@ -1,6 +1,12 @@
-{# Discovered playbook variables as fill-in fields. Shared by the browse run
-   form, the host run form, and the schedule form. Expects `variables` (from
-   discover_playbook_variables). Rendered standalone by /ansible/playbook-vars. #}
+{# Description + discovered variable fields for a selected playbook. Shared by
+   the browse run form, host run form, and schedule form. Expects `variables`
+   and `description`. Rendered standalone by /ansible/playbook-vars. #}
+{% if description %}
+<div style="background:var(--bg);border-left:3px solid var(--accent);border-radius:3px;
+            padding:0.5rem 0.7rem;margin:0.25rem 0 0.6rem;font-size:0.82rem;color:var(--text-muted);">
+  {{ description }}
+</div>
+{% endif %}
 {% if variables %}
 <div style="margin:0.5rem 0 0.25rem;font-size:0.82rem;color:var(--text-muted);font-weight:600;">
   Playbook variables