Use ephemeral SSH keys per rebuild instead of static config keys

Generate a fresh ed25519 key pair at rebuild time, inject the public key
into the Proxmox answer file, use the private key for cluster join over
SSH, then remove the key from both the remote host and the database.
This eliminates the need to manage static SSH keys in config/secrets.

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

README.md

@@ -53,7 +53,6 @@ Edit `provisioning.yaml`:
 - `pxe.subnet` — LAN CIDR for proxy-DHCP
 - `proxmox.existing_node` — IP of any current cluster member
 - `proxmox.join_fingerprint` — from `pvecm status` on an existing node
-- `credentials.ssh_public_key` — public key injected into new hosts
 - `credentials.root_password_hash` — `mkpasswd -m sha-512`
 - `infrastructure.base_url` — URL of the Infrastructure service
 - `infrastructure.server_type_map` — maps local type keys to Infrastructure IDs