feat(mcp): automation tools — playbooks, runs, schedules, alert actions, docker prune

MCP-6 #4970 "Automation tools: playbooks, runs, schedules, alert actions".

Viewer: list_playbooks, get_playbook (secret defaults masked), list_inventory
(variable names only), list_ansible_runs, get_ansible_run (log tail + grep),
list_schedules, wait_for_run (polls, 120s cap).

Operator: run_playbook (check mode unless check=false; a steward:confirm
playbook applies only with confirmed=true; secret variables reach the run
but are never stored, audited or returned), cancel_ansible_run, and
create/update/delete_schedule (secrets refused, check by default).

Admin: set_alert_action, and the docker plugin's prune_docker, which
previews from the disk report unless check=false (Ansible's check mode
skips the prune commands, so a check run would show nothing).

Every write is audited as "<user> via MCP" with the token id; the audit
tool's user filter includes those rows. prune_extra_vars moves to the
docker service so the page and the tool share it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/mcp/tools/session.py

@@ -6,8 +6,10 @@ from steward.models.users import UserRole
 
 _ROLE_MEANS = {
     UserRole.viewer: "Read-only: every list_*/get_* tool. Nothing that changes Steward or your hosts.",
-    UserRole.operator: "Viewer tools, plus running playbooks and changing monitors, alerts and schedules.",
-    UserRole.admin: "Everything, including deleting monitors and alert rules.",
+    UserRole.operator: "Viewer tools, plus running playbooks (check mode unless told otherwise) "
+                       "and changing monitors, alerts and schedules.",
+    UserRole.admin: "Everything, including the playbook an alert rule runs, pruning Docker disk, "
+                    "and deleting monitors and alert rules.",
 }