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/ansible/routes.py

@@ -373,7 +373,6 @@ async def schedules():
         {"name": s["name"], "playbooks": src_module.discover_playbooks(s["path"])}
         for s in _get_sources()
     ]
-    all_playbooks = sorted({p for sd in source_data for p in sd["playbooks"]})
 
     rows = []
     editing = None
@@ -386,6 +385,12 @@ async def schedules():
         if editing_id and s.id == editing_id:
             editing = s
 
+    # Default the source dropdown to the first source (or the edited schedule's
+    # source), and pre-render that source's playbooks so the playbook dropdown
+    # starts on a real item rather than a placeholder.
+    sel_source = editing.source_name if editing else (source_data[0]["name"] if source_data else "")
+    sel_playbooks = next((sd["playbooks"] for sd in source_data if sd["name"] == sel_source), [])
+
     # Edit mode: pre-render the selected playbook's declared variables so their
     # saved values show (fresh loads fetch these via HTMX from /playbook-vars).
     edit_variables: list = []
@@ -401,7 +406,7 @@ async def schedules():
     return await render_template(
         "ansible/schedules.html",
         rows=rows, groups=groups, targets=targets,
-        source_data=source_data, all_playbooks=all_playbooks,
+        source_data=source_data, sel_source=sel_source, sel_playbooks=sel_playbooks,
         editing=editing, interval_presets=INTERVAL_PRESETS,
         edit_variables=edit_variables, edit_description=edit_meta["description"],
         edit_category=edit_meta["category"], edit_confirm=edit_meta["confirm"],