Each device gets a unique ed25519 keypair generated at preconfig time.
Private key goes to Infisical (bigbrother/SSH_PRIVKEY_<ID>), public key
to the SD card only. No operator keys or identifying comments on device.
Copying authorized_keys verbatim onto a drop implant leaks internal hostnames,
usernames, and tool names in key comments. preconfig_sd.sh now strips all
comments (awk '{print $1,$2}') when writing keys to SD card. deploy.sh adds
the same strip step on the live device as a safety net.