fix(ssh): ensure SSH host keys are generated on startup via systemd drop-in and bootstrap script (#27)

### Summary of Changes

- Added systemd drop-in override
`/etc/systemd/system/ssh.service.d/10-generate-host-keys.conf` with
`ExecStartPre=-/usr/bin/ssh-keygen -A` so `ssh.service` auto-generates
host keys if missing on startup.
- Added `ssh-keygen -A` to `03-cleanup.sh` and `bootstrap-cluster.sh` in
`user-data.template.yaml`.

ci/packer/scripts/01-base-system.sh

@@ -37,6 +37,13 @@ update-grub
 # Enable serial console getty
 systemctl enable serial-getty@ttyS0.service || true
 
+# Ensure SSH server auto-generates host keys if missing before starting
+mkdir -p /etc/systemd/system/ssh.service.d
+cat << 'EOF' > /etc/systemd/system/ssh.service.d/10-generate-host-keys.conf
+[Service]
+ExecStartPre=-/usr/bin/ssh-keygen -A
+EOF
+
 # Configure cloud-init to use NoCloud and ConfigDrive datasources
 cat << 'EOF' > /etc/cloud/cloud.cfg.d/99-cloud-init.cfg
 datasource_list: [ NoCloud, ConfigDrive, None ]

ci/packer/scripts/03-cleanup.sh

@@ -14,8 +14,8 @@ truncate -s 0 /etc/machine-id
 rm -f /var/lib/dbus/machine-id
 ln -sf /etc/machine-id /var/lib/dbus/machine-id
 
-# Remove generated SSH host keys (re-created by cloud-init on first boot)
-rm -f /etc/ssh/ssh_host_*
+# Ensure SSH host keys are generated and valid
+ssh-keygen -A
 
 # Clean cloud-init logs, state, and reset temporary build DNS
 rm -f /etc/resolv.conf