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>

steward/templates/ansible/_playbook_vars.html

@@ -1,12 +1,23 @@
 {# Description + discovered variable fields for a selected playbook. Shared by
    the browse run form, host run form, and schedule form. Expects `variables`
    and `description`. Rendered standalone by /ansible/playbook-vars. #}
-{% if description %}
+{% if description or category %}
 <div style="background:var(--bg);border-left:3px solid var(--accent);border-radius:3px;
             padding:0.5rem 0.7rem;margin:0.25rem 0 0.6rem;font-size:0.82rem;color:var(--text-muted);">
+  {% if category %}<span style="display:inline-block;font-size:0.68rem;text-transform:uppercase;letter-spacing:0.04em;
+      background:var(--bg-elevated);color:var(--text-dim);border-radius:3px;padding:0.05em 0.45em;margin-right:0.4rem;">{{ category }}</span>{% endif %}
   {{ description }}
 </div>
 {% endif %}
+{% if confirm %}
+<label style="display:flex;align-items:flex-start;gap:0.5rem;background:color-mix(in srgb,var(--red) 10%,var(--bg-elevated));
+              border:1px solid color-mix(in srgb,var(--red) 35%,var(--border));border-radius:6px;
+              padding:0.6rem 0.8rem;margin:0 0 0.6rem;font-size:0.82rem;font-weight:normal;cursor:pointer;">
+  <input type="checkbox" name="confirmed" required style="margin-top:0.15rem;">
+  <span><strong style="color:var(--red);">Confirm:</strong> this playbook is marked as making
+  significant or destructive changes. Tick to enable running it.</span>
+</label>
+{% endif %}
 {% if variables %}
 <div style="margin:0.5rem 0 0.25rem;font-size:0.82rem;color:var(--text-muted);font-weight:600;">
   Playbook variables