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/hosts/detail.html

@@ -161,9 +161,9 @@
       <div class="form-group">
         <label>Source</label>
         <select name="source_name" required
-                hx-get="/ansible/playbook-options" hx-trigger="change"
-                hx-target="#hp-playbook" hx-swap="innerHTML" hx-include="this">
-          <option value="">— choose source —</option>
+                hx-get="/ansible/playbook-options" hx-trigger="load, change"
+                hx-target="#hp-playbook" hx-swap="innerHTML" hx-include="this"
+                hx-on::after-settle="htmx.trigger('#hp-playbook','change')">
           {% for s in ansible_sources %}<option value="{{ s }}">{{ s }}</option>{% endfor %}
         </select>
       </div>
@@ -172,7 +172,6 @@
         <select name="playbook_path" id="hp-playbook" required
                 hx-get="/ansible/playbook-vars" hx-trigger="change"
                 hx-target="#hp-vars" hx-swap="innerHTML" hx-include="closest form">
-          <option value="">— choose a source first —</option>
         </select>
       </div>
       <div id="hp-vars"></div>