feat(ansible): default source/playbook dropdowns to the first item

The schedule and host run forms opened their Source/Playbook <select>s on a
"— choose … —" placeholder, forcing an extra click. Default them to the first
item and cascade the dependent fields so the initial state is real:

- Remove the placeholder options (schedules source+playbook, host source+
  playbook, and the shared _playbook_options fragment).
- schedules(): pass sel_source (first source, or the edited schedule's) and
  that source's pre-rendered playbooks so the playbook dropdown starts populated
  (create mode too, no longer the all-sources union).
- Cascade on load/change: the source <select> fires playbook-options then
  (hx-on::after-settle) re-triggers the playbook <select>, which loads that
  playbook's variable fields. Host form drives the whole chain from a load
  trigger; schedule create mode loads vars via the playbook <select>'s own load
  trigger, while edit mode keeps its server-rendered prefill untouched.

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

steward/templates/ansible/_playbook_options.html

@@ -1,6 +1,7 @@
 {# <option> list for a playbook <select>, populated when a source is chosen.
-   Returned by /ansible/playbook-options. `selected` pre-selects one (edit). #}
-<option value="">— choose playbook —</option>
+   Returned by /ansible/playbook-options. `selected` pre-selects one; with none
+   given the browser selects the first, so the dropdown never sits on a
+   placeholder. #}
 {% for pb in playbooks %}
 <option value="{{ pb }}" {% if pb == selected %}selected{% endif %}>{{ pb }}</option>
 {% endfor %}