fix(ansible): write DB inventory as static YAML, not dynamic --list JSON

generate_inventory() emits Ansible's dynamic --list shape (all.hosts is a
LIST, vars under _meta) — valid only as an executable inventory script's
stdout. We were writing it to a static file and passing -i, so Ansible's yaml
plugin rejected it ("Invalid 'hosts' entry for 'all' group, requires a
dictionary, found ...list...") and fell back to implicit localhost → "no hosts
matched". Affected every steward:* scope run; surfaced on the first real
provision.

- New inventory_to_yaml(inv): convert the --list dict → a valid static YAML
  inventory (all.hosts dict keyed by host, groups under all.children, group
  vars preserved, injected per-host vars like steward_token retained).
- Wire it into runner.trigger_run, host_agent deploy + provision.
- executor writes the file as inventory.yml so the yaml plugin's extension
  check reliably claims it.
- generate_inventory unchanged (still the --list dict); conversion happens at
  write time. Unit tests added.

Scribe issue #885.

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

steward/ansible/executor.py

@@ -375,7 +375,9 @@ async def start_run(
         os.chmod(tmpdir, 0o700)
         try:
             if inventory_content:
-                inv_target = os.path.join(tmpdir, "inventory")
+                # .yml so Ansible's yaml inventory plugin reliably claims it
+                # (the plugin's verify_file checks the extension).
+                inv_target = os.path.join(tmpdir, "inventory.yml")
                 with open(inv_target, "w", encoding="utf-8") as invf:
                     invf.write(inventory_content)
             else: