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/ansible/routes.py

@@ -143,6 +143,35 @@ def _parse_run_params(form) -> tuple[dict | None, dict | None, str | None]:
     return (params or None), (secret_vars or None), None
 
 
+@ansible_bp.get("/playbook-options")
+@require_role(UserRole.operator)
+async def playbook_options():
+    """HTMX fragment: <option>s of playbooks in a source, to populate a playbook
+    dropdown when a source is chosen. `selected` pre-selects one (edit forms)."""
+    source_name = (request.args.get("source_name", "") or "").strip()
+    selected = (request.args.get("selected", "") or "").strip()
+    source = next((s for s in _get_sources() if s["name"] == source_name), None)
+    playbooks = src_module.discover_playbooks(source["path"]) if source else []
+    return await render_template(
+        "ansible/_playbook_options.html", playbooks=playbooks, selected=selected)
+
+
+@ansible_bp.get("/playbook-vars")
+@require_role(UserRole.operator)
+async def playbook_vars():
+    """HTMX fragment: fill-in fields for a playbook's declared variables
+    (vars/vars_prompt), loaded when a playbook is chosen from a dropdown."""
+    source_name = (request.args.get("source_name", "") or "").strip()
+    playbook_path = (request.args.get("playbook_path", "") or "").strip()
+    source = next((s for s in _get_sources() if s["name"] == source_name), None)
+    variables: list = []
+    if source and playbook_path:
+        contents = src_module.read_playbook(source["path"], playbook_path)
+        if contents is not None:
+            variables = src_module.discover_playbook_variables(contents)
+    return await render_template("ansible/_playbook_vars.html", variables=variables)
+
+
 @ansible_bp.get("/run-form/<source_name>/<path:playbook_path>")
 @require_role(UserRole.operator)
 async def run_form(source_name: str, playbook_path: str):