fix: bake STACKS_DIR into systemd service units at install time

The update and backup services ran without STACKS_DIR since systemd
doesn't source ~/.env or ~/.zshrc. Capture the value at install time
and write it as Environment= in the generated unit files.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bin/stacks

@@ -475,6 +475,7 @@ Type=oneshot
 User=root
 ExecStart=${_self} update run
 Environment=HOME=/root
+Environment=STACKS_DIR=${STACKS_DIR}
 EOF
       cat > /etc/systemd/system/stacks-update.timer << 'EOF'
 [Unit]
@@ -632,6 +633,7 @@ Type=oneshot
 User=root
 ExecStart=${_self} backup run
 Environment=HOME=/root
+Environment=STACKS_DIR=${STACKS_DIR}
 EOF
       cat > /etc/systemd/system/stacks-backup.timer << 'EOF'
 [Unit]