Add boot-time secrets loading service to prevent service failures after reboot

- Add aitbc-load-secrets.service to load /run/aitbc/secrets/.env at boot
- Add aitbc-secrets.conf tmpfiles.d config to create secrets directory
- Update link-systemd.sh to deploy tmpfiles.d configurations
- Fixes issue where services fail with 'Failed to load environment files' after reboot

scripts/config/aitbc-secrets.conf

@@ -0,0 +1,4 @@
+# Create runtime directory for AITBC secrets at boot
+# This directory is in tmpfs (/run) and gets cleared on reboot
+# Deployed to /etc/tmpfiles.d/ by setup scripts
+d /run/aitbc/secrets 0700 root root -