Generate a nonce for boot attestation.

This endpoint is called by VMs during boot before any registration. No authentication required as the VM doesn't exist in the system yet.

miner_hotkey is OPTIONAL for backwards compatibility: older VM initramfs fetch the nonce without it. When supplied it is bound into the nonce, and boot attestation enforces that args.miner_hotkey matches the value stored here (preventing cross-miner nonce reuse). VMs that omit it get an unbound nonce and the binding check is skipped.

TODO: make miner_hotkey required when all VMs >= 1.4.0