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>

README.md

@@ -1,6 +1,6 @@
 ---
 created: 2026-04-06T09:52
-updated: 2026-04-07T21:59
+updated: 2026-04-13T10:10
 ---
 # MajorLinux Tech Wiki — Index
 
@@ -142,6 +142,7 @@ updated: 2026-04-07T21:59
 - [Gemini CLI Manual Update](05-troubleshooting/gemini-cli-manual-update.md) — how to manually update the Gemini CLI when automatic updates fail
 - [MajorWiki Setup & Pipeline](05-troubleshooting/majwiki-setup-and-pipeline.md) — setting up MajorWiki and the Obsidian → Gitea → MkDocs publishing pipeline
 - [Gitea Actions Runner: Boot Race Condition Fix](05-troubleshooting/gitea-runner-boot-race-network-target.md) — fixing act_runner crash loop on boot caused by DNS not ready at startup
+- [Cron Heartbeat False Alarm: /var/run Cleared by Reboot](05-troubleshooting/cron-heartbeat-tmpfs-reboot-false-alarm.md) — why `/run` is tmpfs and how a reboot wipes cron heartbeat files, and where to put them instead
 - [SELinux: Fixing Dovecot Mail Spool Context (/var/vmail)](05-troubleshooting/selinux-dovecot-vmail-context.md) — fixing thousands of AVC denials when /var/vmail has wrong SELinux context
 - [mdadm RAID Recovery After USB Hub Disconnect](05-troubleshooting/storage/mdadm-usb-hub-disconnect-recovery.md) — diagnosing and recovering a failed mdadm array caused by a USB hub dropout
 - [Windows OpenSSH Server (sshd) Stops After Reboot](05-troubleshooting/networking/windows-sshd-stops-after-reboot.md) — fixing sshd not running after reboot due to Manual startup type
@@ -160,6 +161,7 @@ updated: 2026-04-07T21:59
 
 | Date | Article | Domain |
 |---|---|---|
+| 2026-04-13 | [Cron Heartbeat False Alarm: /var/run Cleared by Reboot](05-troubleshooting/cron-heartbeat-tmpfs-reboot-false-alarm.md) | Troubleshooting |
 | 2026-04-08 | [wget/curl: URLs with Special Characters Fail in Bash](05-troubleshooting/wget-url-special-characters.md) | Troubleshooting |
 | 2026-04-07 | [SSH Config & Key Management](01-linux/networking/ssh-config-key-management.md) | Linux |
 | 2026-04-07 | [Windows OpenSSH: WSL Default Shell Breaks Remote Commands](05-troubleshooting/networking/windows-openssh-wsl-default-shell-breaks-remote-commands.md) | Troubleshooting |