nixos/portcullis: Add initial config

New bare-metal box headed for Nikhef, intended to take over most of
estuary's colony edge routing. This is the bootstrap config only: the
hardware, the single-NVMe ESP + LVM layout, and enough networking to
boot and be reachable.

It is being staged at home before it is racked, so it has no colony
assignments yet. Every 2.5G port takes DHCP and whichever one is
patched in brings the box up; kea registers the DHCP hostname, so the
deploy node points at portcullis.dyn.h.nul.ie until there is a real
colony FQDN for it.

The host key was adopted from the installer session and seeded onto
the persist volume before first boot, so my.secrets.key could be set
up front -- which makes portcullis a recipient of the user-passwd
secret that my.user declares for every box.

Documented with a box page, a row in the colony site index, and a note
in the colony section of networking.md that the topology is expected
to change once portcullis takes over from estuary.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/sites/colony/README.md

@@ -24,9 +24,13 @@ prefixes and routing overview are in the [`colony` section of networking.md](../
 | [`git`](git.md) | Gitea + Gitea Actions runner |
 | [`mail`](mail.md) | Debian VM running mailcow (not NixOS) |
 | [`darts`](darts.md) | Third-party/customer VM (not NixOS) |
+| [`portcullis`](portcullis.md) | Bare-metal edge box for Nikhef; being staged, not yet in service |
 
 The applications running on `shill` are listed on its own page — see
 [shill/README.md](shill/README.md#containers).
 
 `mail` and `darts` are host-defined VMs whose guest operating systems are managed out of band; their
 pages document only what this repository controls.
+
+`portcullis` is new hardware headed for Nikhef that will take over most of `estuary`'s edge routing.
+It is not deployed yet and the resulting topology is still being worked out.