feat(ansible): dropdown playbook selection + auto-populated variable fields

Stop making operators type playbook paths and guess extra-var names.

- Reusable infra: shared ansible/_playbook_vars.html (the discovered-variable
  fields) + ansible/_playbook_options.html; two HTMX endpoints —
  /ansible/playbook-options (a source's playbooks, optional ?selected for edit)
  and /ansible/playbook-vars (a playbook's vars:/vars_prompt: as fill-in
  fields). browse _run_form.html refactored to include the shared partial.
- Host "Run a playbook against this host": source dropdown → playbook dropdown
  → variable fields, all chained via HTMX. Handler reuses _parse_run_params so
  var__/secret__ fields flow through extra_vars_map + the unpersisted
  secret_vars channel.
- Schedules: playbook free-text+datalist → source-dependent dropdown; fixed the
  extra-vars edit pre-fill to read extra_vars_map (stale list key after the
  earlier JSON-extra-vars change).

Scribe #895.

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

steward/templates/ansible/schedules.html

@@ -86,7 +86,9 @@
       </div>
       <div class="form-group" style="margin-bottom:0;">
         <label>Source</label>
-        <select name="source_name" required>
+        <select name="source_name" required
+                hx-get="/ansible/playbook-options" hx-trigger="change"
+                hx-target="#sch-playbook" hx-swap="innerHTML" hx-include="this">
           <option value="">— choose source —</option>
           {% for sd in source_data %}
           <option value="{{ sd.name }}" {% if editing and editing.source_name == sd.name %}selected{% endif %}>{{ sd.name }}</option>
@@ -94,12 +96,13 @@
         </select>
       </div>
       <div class="form-group" style="margin-bottom:0;">
-        <label>Playbook path</label>
-        <input type="text" name="playbook_path" required list="playbooks"
-               value="{{ editing.playbook_path if editing else '' }}" placeholder="maintenance/docker_prune.yml">
-        <datalist id="playbooks">
-          {% for pb in all_playbooks %}<option value="{{ pb }}"></option>{% endfor %}
-        </datalist>
+        <label>Playbook</label>
+        <select name="playbook_path" id="sch-playbook" required>
+          <option value="">— choose playbook —</option>
+          {% for pb in all_playbooks %}
+          <option value="{{ pb }}" {% if editing and editing.playbook_path == pb %}selected{% endif %}>{{ pb }}</option>
+          {% endfor %}
+        </select>
       </div>
       <div class="form-group" style="margin-bottom:0;">
         <label>Target</label>
@@ -132,7 +135,8 @@
     </div>
     <div class="form-group" style="margin-top:1rem;">
       <label>Extra vars (one key=value per line)</label>
-      <textarea name="extra_vars" rows="3" placeholder="prune_volumes=false">{{ p.get('extra_vars', []) | join('\n') }}</textarea>
+      <textarea name="extra_vars" rows="3" placeholder="prune_volumes=false">{% for k, v in (p.get('extra_vars_map') or {}).items() %}{{ k }}={{ v }}
+{% endfor %}</textarea>
     </div>
     <div class="form-group" style="display:flex;align-items:center;gap:0.5rem;">
       <input type="checkbox" name="check" id="check" style="width:auto;" {% if p.get('check') %}checked{% endif %}>