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/host_agent/install.yml

@@ -1,4 +1,5 @@
 ---
+# description: Install or re-enroll the Steward host agent on an already-provisioned host.
 # Install (or update) the Steward host agent on a target. Mirrors the
 # curl-based install.sh. Pass the per-host registration token as extra-vars:
 #   steward_url=https://steward.example  steward_token=<token from Host Agents>