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/britnet.md

@@ -0,0 +1,21 @@
+# britnet
+
+A VPS in Birmingham (`bhx1`) acting as a Tailscale/WireGuard gateway node.
+
+- **Source:** [`nixos/boxes/britnet.nix`](../../../nixos/boxes/britnet.nix)
+- **Internal domain:** `bhx1.int.nul.ie`
+
+## Role
+
+- **Tailscale node** + **WireGuard** (`wg0`) gateway: provides a second egress /
+  entry point into the boxes' overlay networks.
+- nftables SNATs traffic arriving on `tailscale0` / `wg0` out of the provider
+  interface (`veth0`), using the `allhost` assignment addresses.
+
+## Networking
+
+- Provider uplink with gateways `77.74.199.1` (v4) / `2a12:ab46:5344::1` (v6).
+- `tailscale0` and `wg0` overlay interfaces; `allhost` assignment for SNAT.
+
+> `britnet` is a separate machine from [`britway`](britway.md) — different
+> provider/site, narrower role (gateway rather than control plane + BGP edge).