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>

05-troubleshooting/index.md

@@ -1,6 +1,6 @@
 ---
 created: 2026-03-15T06:37
-updated: 2026-04-08
+updated: 2026-04-13T10:10
 ---
 # 🔧 General Troubleshooting
 
@@ -29,6 +29,7 @@ Practical fixes for common Linux, networking, and application problems.
 - [Gitea Actions Runner: Boot Race Condition Fix](gitea-runner-boot-race-network-target.md)
 - [Systemd Session Scope Fails at Login (`session-cN.scope`)](systemd/session-scope-failure-at-login.md)
 - [MajorWiki Setup & Publishing Pipeline](majwiki-setup-and-pipeline.md)
+- [Cron Heartbeat False Alarm: /var/run Cleared by Reboot](cron-heartbeat-tmpfs-reboot-false-alarm.md)
 
 ## 🔒 SELinux
 - [SELinux: Fixing Dovecot Mail Spool Context (/var/vmail)](selinux-dovecot-vmail-context.md)