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/hosts/detail.html

@@ -109,23 +109,38 @@
     <form method="post" action="/hosts/{{ host.id }}/run-playbook" style="margin-top:0.75rem;">
       <div class="form-group">
         <label>Source</label>
-        <select name="source_name" required>
+        <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>
           {% for s in ansible_sources %}<option value="{{ s }}">{{ s }}</option>{% endfor %}
         </select>
       </div>
       <div class="form-group">
-        <label>Playbook <span style="color:var(--text-muted);font-weight:normal;">(path within source)</span></label>
-        <input type="text" name="playbook_path" placeholder="playbooks/site.yml" required>
-      </div>
-      <div class="form-group">
-        <label>Extra vars <span style="color:var(--text-muted);font-weight:normal;">(one key=value per line)</span></label>
-        <textarea name="extra_vars" rows="2" style="width:100%;font-family:ui-monospace,monospace;font-size:0.85rem;"></textarea>
-      </div>
-      <div class="form-group">
-        <label style="display:flex;align-items:center;gap:0.5rem;font-weight:normal;">
-          <input type="checkbox" name="check"> Dry-run (--check --diff)
-        </label>
+        <label>Playbook</label>
+        <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>
+      <details style="margin:0.25rem 0 0.75rem;">
+        <summary style="cursor:pointer;font-size:0.82rem;color:var(--text-muted);">Advanced</summary>
+        <div class="form-group" style="margin-top:0.6rem;">
+          <label>Extra vars <span style="color:var(--text-muted);font-weight:normal;">(one key=value per line)</span></label>
+          <textarea name="extra_vars" rows="2" style="width:100%;font-family:ui-monospace,monospace;font-size:0.85rem;"></textarea>
+        </div>
+        <div class="form-group">
+          <label>Tags <span style="color:var(--text-muted);font-weight:normal;">(--tags)</span></label>
+          <input type="text" name="tags" placeholder="deploy,config">
+        </div>
+        <div class="form-group">
+          <label style="display:flex;align-items:center;gap:0.5rem;font-weight:normal;">
+            <input type="checkbox" name="check"> Dry-run (--check --diff)
+          </label>
+        </div>
+      </details>
       <button type="submit" class="btn btn-sm">Run playbook</button>
     </form>
   </details>