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/_playbook_vars.html

@@ -0,0 +1,32 @@
+{# Discovered playbook variables as fill-in fields. Shared by the browse run
+   form, the host run form, and the schedule form. Expects `variables` (from
+   discover_playbook_variables). Rendered standalone by /ansible/playbook-vars. #}
+{% if variables %}
+<div style="margin:0.5rem 0 0.25rem;font-size:0.82rem;color:var(--text-muted);font-weight:600;">
+  Playbook variables
+</div>
+<p style="color:var(--text-dim);font-size:0.76rem;margin:0 0 0.6rem;">
+  Leave a field blank to use the playbook/inventory default. Secret fields are never stored.
+</p>
+{% for v in variables %}
+<div class="form-group">
+  <label>
+    {{ v.prompt or v.name }}
+    <span style="color:var(--text-muted);font-weight:normal;font-size:0.76rem;">
+      ({{ v.name }}{% if v.required %}, required{% endif %}{% if v.secret %}, secret{% endif %})
+    </span>
+  </label>
+  {% if v.secret %}<input type="hidden" name="secret__{{ v.name }}" value="1">{% endif %}
+  <input type="{{ 'password' if v.secret else 'text' }}"
+         name="var__{{ v.name }}"
+         {% if v.required %}required{% endif %}
+         {% if v.secret %}autocomplete="new-password"{% endif %}
+         placeholder="{% if v.secret %}(hidden){% elif v.default not in (None, '') %}default: {{ v.default }}{% else %}no default{% endif %}"
+         style="width:100%;font-family:ui-monospace,monospace;font-size:0.85rem;">
+</div>
+{% endfor %}
+{% else %}
+<p style="color:var(--text-dim);font-size:0.8rem;margin:0.25rem 0 0.6rem;">
+  No declared variables found in this playbook.
+</p>
+{% endif %}