Join the tailnet from a cidata tailscale_authkey

A tailscale_authkey file on the autoinstall drive gets the installed
machine onto the tailnet on its own: the tailscale package is bundled in
the offline mirror and installed only when the key is present, the key is
staged root-only at /etc/tailscale/authkey, and a oneshot first-boot unit
runs tailscale up once the network is really there. On success the key is
deleted and the unit disables itself; on failure both survive to retry on
the next boot, so an offline install joins whenever it first gets
connectivity. ufw allows traffic in on tailscale0 via the same
write-the-rule-then-ignore-the-chroot-exit-code dance as the SSH phase.

Bundling rather than fetching at first boot is deliberate: pacman -Sy
tailscale on a fresh machine is a partial upgrade, and pacman -Syu means
an unattended full system upgrade racing the user's first login. The
offline mirror is a coherent snapshot; installing from it keeps the
target consistent and the install offline-capable. See plans/tailscale.md.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Xc74h4WSYpuYPGuYbFAR1q

builder/archinstall.packages

@@ -12,3 +12,4 @@ openssh
 pipewire
 snapper
 sof-firmware
+tailscale