feat(security): add SSH hardening role

Port Modules/Security/secharden-ssh.sh.

Deploys:
- Authorized keys for root, localuser, subodev (conditional on existence)
- Hardened /etc/ssh/sshd_config (skipped on dev workstations)
- ssh-audit hardening drop-in for algorithm restrictions (skipped on
  Ubuntu where it breaks openssh-server)
- Lockdown of sshd_config.d directory permissions

SSH service is validated (sshd -t) before restart.

🤖 Generated with [Crush](https://github.com/charmassociates/crush)

Assisted-by: GLM-5 via Crush <crush@charm.land>

roles/security_ssh/files/configs/sshd_config

@@ -0,0 +1,20 @@
+Include /etc/ssh/sshd_config.d/*.conf
+HostKey /etc/ssh/ssh_host_rsa_key
+HostKey /etc/ssh/ssh_host_ed25519_key
+KbdInteractiveAuthentication no
+PrintMotd no
+PasswordAuthentication no
+AllowTcpForwarding no
+X11Forwarding no
+ChallengeResponseAuthentication no
+AcceptEnv LANG LC_*
+Subsystem       sftp    /usr/lib/openssh/sftp-server
+UsePAM yes
+Banner /etc/issue.net
+MaxAuthTries 2
+MaxStartups 10:30:100
+PermitRootLogin prohibit-password
+ClientAliveInterval 300
+ClientAliveCountMax 3
+AllowUsers root localuser subodev
+LoginGraceTime 60