docs: Document the boxes

Add a top-level `README.md` mapping the boxes and per-machine docs under
`docs/boxes/` (grouped `colony/`, `home/`, `misc/`), one file per host, VM and
container documenting role, services and networking with source pointers.

Also point `AGENTS.md` at the new docs.

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

docs/boxes/misc/britway.md

@@ -0,0 +1,27 @@
+# britway
+
+A Vultr VPS in London (`lon1`) acting as a network edge node: the Tailscale
+control plane, an exit node, and a BGP speaker in the AS211024 mesh.
+
+- **Source:** [`nixos/boxes/britway/`](../../../nixos/boxes/britway)
+  (`default.nix`, `bgp.nix`, `nginx.nix`, `tailscale.nix`)
+- **Internal domain:** `lon1.int.nul.ie`
+
+## Role
+
+- **Headscale** ([`tailscale.nix`](../../../nixos/boxes/britway/tailscale.nix)) — the
+  self-hosted Tailscale control server (`hs.nul.ie`) the rest of the boxes log
+  into.
+- **Tailscale node** — advertises itself as an **exit node** and advertises the
+  tailnet routes, so tailnet clients can egress / reach internal prefixes via
+  britway.
+- **BGP** ([`bgp.nix`](../../../nixos/boxes/britway/bgp.nix)) — part of the AS211024
+  L2 VXLAN mesh (`my.vpns.l2`) alongside `estuary`, `river` and `stream`.
+- **nginx** ([`nginx.nix`](../../../nixos/boxes/britway/nginx.nix)) — reverse proxy /
+  web front-end with ACME certs.
+
+## Networking
+
+- `vultr` assignment on the provider interface; `as211024` on the mesh.
+- A `veth0`/`tailscale0` setup with SNAT so tailnet traffic egresses via the VPS
+  public IP.