feat(ansible): steward:category + steward:confirm playbook metadata

Extend the playbook metadata convention with a namespaced `# steward:<key>:`
comment block:

- steward:category — free-text grouping label, shown as a badge in the browse
  list and on the run form.
- steward:confirm — true/yes/1/on marks a playbook destructive; the run form
  then requires a confirmation tick (required checkbox in the shared vars
  fragment) before it can launch.

sources.discover_playbook_meta() parses description + category + confirm (first
match per key; `# description:` still primary, `# steward:description:` alias).
discover_playbook_description() now delegates to it. The browse list reads
per-playbook meta to show category badges + descriptions; the run-form and
playbook-vars fragments render the badge + confirm gate.

Bundled playbooks tagged: docker_prune → category maintenance + confirm true;
provision/install/update → category host-agent.

Docs: docs/reference/playbook-authoring.md updated (keys now implemented) and a
quick reference added next to the code at steward/ansible/PLAYBOOK_CONVENTIONS.md.
Tests added for category/confirm/alias parsing.

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

tests/test_playbook_variables.py

@@ -4,6 +4,7 @@ import json
 from steward.ansible.executor import build_extra_vars_file
 from steward.ansible.sources import (
     discover_playbook_description,
+    discover_playbook_meta,
     discover_playbook_variables,
 )
 
@@ -137,3 +138,31 @@ def test_description_falls_back_to_play_name():
 def test_description_empty_when_neither_present():
     assert discover_playbook_description("- hosts: all\n  tasks: []\n") == ""
     assert discover_playbook_description("not: : valid") == ""
+
+
+# ── steward:category / steward:confirm metadata ───────────────────────────────
+
+def test_meta_category_and_confirm():
+    content = """---
+# description: Prune docker.
+# steward:category: maintenance
+# steward:confirm: true
+- hosts: all
+  tasks: []
+"""
+    meta = discover_playbook_meta(content)
+    assert meta["description"] == "Prune docker."
+    assert meta["category"] == "maintenance"
+    assert meta["confirm"] is True
+
+
+def test_meta_confirm_falsey_and_absent():
+    assert discover_playbook_meta("# steward:confirm: false\n- hosts: all")["confirm"] is False
+    assert discover_playbook_meta("- hosts: all\n  name: x")["confirm"] is False
+    assert discover_playbook_meta("- hosts: all\n  name: x")["category"] == ""
+
+
+def test_meta_steward_description_alias():
+    # `# steward:description:` works when there's no plain `# description:`.
+    meta = discover_playbook_meta("# steward:description: via namespace\n- hosts: all")
+    assert meta["description"] == "via namespace"