fix(alerting): dead-man heartbeat never pushed — secret unavailable in runtime templates

HA runtime template contexts (shell_command {{ }} rendering) cannot read
secrets.yaml; kuma_push_url rendered undefined every fire and the Kuma
push dead-man never sent a beat. Move the full curl command into the
secret (kuma_heartbeat_cmd) and reference it with !secret, which is
substituted at config parse where secrets work.

On-box fix applied + verified under GLPI CR 21; this commit makes the
git copy match the deployed state.

Ticket: https://projects.knownelement.com/issues/811
CR: https://cmdb.knownelement.com/front/change.form.php?id=21

packages/deploy_pipeline.yaml

@@ -25,4 +25,4 @@ rest:
 # pushing — crash, wedge, power loss, network partition — Kuma pages
 # the founder's phone directly. No HA in the alert loop.
 shell_command:
-  kuma_ha_heartbeat: 'curl -sS -m 15 "{{ kuma_push_url }}"'
+  kuma_ha_heartbeat: !secret kuma_heartbeat_cmd  # CR21 2026-09-06: runtime template contexts cannot read secrets; the old {{ }} form rendered undefined and the dead-man never pushed