feat(image): remote LUKS unlock via a Tor onion service in the initramfs

Encrypted image setups can now bake a Tor onion service into the initramfs
so the dropbear unlock shell stays reachable behind NAT or a dynamic IP.
When the user opts in, configure_encryption installs tor + busybox, drops
the mkinitcpio hooks (ordered `netconf tor dropbear encryptssh`), generates
the v3 onion keys offline in the image chroot, and prints the stable
.onion address. Unlock with `torsocks ssh root@<onion-address>`.

The runtime hook syncs the clock via NTP first (RTC-less boards boot at
1970, which Tor's consensus checks reject) and starts the onion service
pointing at dropbear on 127.0.0.1:22.

Hardening baked in from an adversarial review of the shipped path:
- cmdline.txt boot path (RPi4-class firmware boot) now sets the same
  ip=::::<host>:eth0:dhcp net.ifnames=0 params as the boot.txt path, so
  the initramfs actually gets a network and the onion can publish.
- the initramfs bakes in libnss_dns.so.2 so the NTP hostname resolves.
- the hook extracts DHCP DNS with sed instead of sourcing the lease files,
  which would run attacker-controlled DHCP option strings as root pre-boot.
- NTP is attempted unconditionally (bounded), not gated on DHCP-provided DNS.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lim/image/plan.py

@@ -17,6 +17,7 @@ class ImagePlan:
     luks_memory_cost: str | None = None
     raspberry_pi_version: str | None = None
     encrypt_system: bool = False
+    tor_unlock: bool = False
     root_filesystem: str = ""
     image_folder: Path = field(default_factory=Path)