Document reverse_tunnel_key creation for the SSH reverse tunnel post (#139)

[simlarsen]

The SSH reverse tunnel post referenced /home/tunnel-user/.ssh/reverse_tunnel_key in the autossh systemd unit but never explained how to create it (#139).

This adds a "Creating the Tunnel Key" section before the systemd unit, walking less-experienced readers through:

• Generating a dedicated ed25519 keypair at the exact path the unit expects: ssh-keygen -t ed25519 -f /home/tunnel-user/.ssh/reverse_tunnel_key -N "".
• Setting correct ownership and permissions (.ssh 700, private key 600).
• Installing the public key into the remote tunnel@203.0.113.10 account's authorized_keys with reverse-forwarding-only restrictions (restrict,port-forwarding,permitlisten="127.0.0.1:2222").
• Pre-seeding the server's host key into known_hosts so the unattended service works with StrictHostKeyChecking=yes.

All commands and paths match names already used in the post. Verified against the OpenSSH man pages for ssh-keygen, sshd, and ssh-keyscan. npm run validate passes.

Resolves #139