feat(ansible): parameterize docker_prune with prune_target (containers/images/system)

M78 step 1. The bundled maintenance/docker_prune.yml was a single
`docker system prune -f`; the upcoming per-host prune buttons need a
granular split. Add a `prune_target` extra-var:
- containers → docker container prune -f
- images     → docker image prune -f (+ -a when prune_all_images)
- system     → docker system prune -f (+ -a / --volumes) — the DEFAULT,
               so existing manual/scheduled callers (which set no var)
               keep today's behavior.

Reuses the existing bundled playbook rather than adding parallel files;
keeps the description/category/confirm meta so it still self-describes as
a confirm-gated maintenance run. Adds unit coverage: the playbook stays
valid YAML, its meta still discovers, and all three targets are guarded.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CAGR73DUowdVFVvYzLXC5C

steward/ansible/bundled/maintenance/docker_prune.yml

@@ -1,29 +1,64 @@
 ---
-# description: Reclaim disk on Docker / Swarm nodes by pruning unused images, containers, networks and build cache.
+# description: Reclaim disk on Docker / Swarm nodes — prune stopped containers, unused images, or a full system prune.
 # steward:category: maintenance
 # steward:confirm: true
 # Reclaim disk on Docker / Docker Swarm nodes by removing unused data.
-# Safe by default: prunes dangling images, stopped containers, unused networks
-# and build cache. Set extra-vars to widen scope:
-#   prune_all_images=true   also remove ALL unused images (not just dangling)
-#   prune_volumes=true      also remove unused named volumes (data loss risk)
-- name: Docker system prune
+# `prune_target` selects the scope (default `system` preserves the original
+# behavior for existing manual/scheduled callers that don't set it):
+#   prune_target=containers   remove stopped containers only (docker container prune)
+#   prune_target=images       remove unused images (docker image prune;
+#                             + prune_all_images=true → -a, i.e. ALL unused, not just dangling)
+#   prune_target=system       docker system prune (dangling images, stopped
+#                             containers, unused networks, build cache). Widen with:
+#                               prune_all_images=true   also ALL unused images
+#                               prune_volumes=true       also unused named volumes (data loss risk)
+- name: Docker prune
   hosts: all
   gather_facts: false
   become: true
   vars:
+    prune_target: system
     prune_all_images: false
     prune_volumes: false
   tasks:
+    - name: Validate prune_target
+      ansible.builtin.assert:
+        that: prune_target in ['containers', 'images', 'system']
+        fail_msg: "prune_target must be one of: containers, images, system (got '{{ prune_target }}')"
+        quiet: true
+
+    # ── Stopped containers only ───────────────────────────────────────────────
+    - name: Prune stopped containers
+      ansible.builtin.command:
+        argv: ['docker', 'container', 'prune', '-f']
+      register: container_prune
+      changed_when: "'Total reclaimed space: 0B' not in container_prune.stdout"
+      when: prune_target == 'containers'
+
+    # ── Unused images (dangling, or all unused with -a) ───────────────────────
+    - name: Prune unused images
+      ansible.builtin.command:
+        argv: >-
+          {{ ['docker', 'image', 'prune', '-f']
+             + (['-a'] if prune_all_images | bool else []) }}
+      register: image_prune
+      changed_when: "'Total reclaimed space: 0B' not in image_prune.stdout"
+      when: prune_target == 'images'
+
+    # ── Full system prune (default) ───────────────────────────────────────────
     - name: Run docker system prune
       ansible.builtin.command:
         argv: >-
           {{ ['docker', 'system', 'prune', '-f']
              + (['-a'] if prune_all_images | bool else [])
              + (['--volumes'] if prune_volumes | bool else []) }}
-      register: prune_result
-      changed_when: "'Total reclaimed space: 0B' not in prune_result.stdout"
+      register: system_prune
+      changed_when: "'Total reclaimed space: 0B' not in system_prune.stdout"
+      when: prune_target == 'system'
 
     - name: Report reclaimed space
       ansible.builtin.debug:
-        msg: "{{ prune_result.stdout_lines | select | list }}"
+        msg: >-
+          {{ ((container_prune.stdout_lines | default([]))
+              + (image_prune.stdout_lines | default([]))
+              + (system_prune.stdout_lines | default([]))) | select | list }}