wiki: add troubleshooting article on /var/run heartbeat reboot false alarm

Captures the majorlab incident where the backup watchdog emailed a missing
heartbeat after a kernel-update reboot wiped /var/run, even though the
backup had actually completed cleanly. Documents the tmpfs root cause and
the fix of storing heartbeats under /var/lib instead.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

SUMMARY.md

@@ -1,6 +1,6 @@
 ---
 created: 2026-04-02T16:03
-updated: 2026-04-08
+updated: 2026-04-13T10:10
 ---
 * [Home](index.md)
 * [Linux & Sysadmin](01-linux/index.md)
@@ -69,6 +69,7 @@ updated: 2026-04-08
     * [Gemini CLI Manual Update](05-troubleshooting/gemini-cli-manual-update.md)
     * [MajorWiki Setup & Publishing Pipeline](05-troubleshooting/majwiki-setup-and-pipeline.md)
     * [Gitea Actions Runner: Boot Race Condition Fix](05-troubleshooting/gitea-runner-boot-race-network-target.md)
+    * [Cron Heartbeat False Alarm: /var/run Cleared by Reboot](05-troubleshooting/cron-heartbeat-tmpfs-reboot-false-alarm.md)
     * [SELinux: Fixing Dovecot Mail Spool Context (/var/vmail)](05-troubleshooting/selinux-dovecot-vmail-context.md)
     * [mdadm RAID Recovery After USB Hub Disconnect](05-troubleshooting/storage/mdadm-usb-hub-disconnect-recovery.md)
     * [Windows OpenSSH Server (sshd) Stops After Reboot](05-troubleshooting/networking/windows-sshd-stops-after-reboot.md)