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/tower.md

@@ -0,0 +1,16 @@
+# tower
+
+A laptop workstation — a Framework Laptop 13 (12th-gen Intel).
+
+- **Source:** [`nixos/boxes/tower/default.nix`](../../../nixos/boxes/tower/default.nix)
+
+## Role
+
+- Framework Laptop 13 (12th-gen Intel) running the GUI environment
+  (`my.gui.enable`) with home-manager on the `mine` channel.
+- Joins the tailnet via the self-hosted Headscale on [`britway`](britway.md)
+  (`tailscale up --login-server=https://hs.nul.ie --accept-routes`).
+- Local virtualisation enabled (`kvm-intel`, IOMMU on).
+
+> Unlike [`castle`](../home/castle.md), `tower` lives outside the `home/` box
+> tree and boots from local disks rather than NVMe-oF.