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/home/castle.md

@@ -0,0 +1,19 @@
+# castle
+
+A desktop workstation.
+
+- **Source:** [`nixos/boxes/home/castle/default.nix`](../../../nixos/boxes/home/castle/default.nix)
+
+## Role
+
+- AMD desktop running the GUI environment (`my.gui.enable`, Sway / Wayland via
+  home-manager).
+- **Diskless-style boot:** its `/nix`, `/persist` and `/home` are NVMe-oF volumes
+  served by [`cellar`](cellar.md) (`/dev/nvmeof/{nix,persist,home}`). The
+  networking is careful not to drop the IP used for the NVMe-oF connection.
+- Uses the `my.nvme` module for the NVMe-oF client setup.
+
+## Networking
+
+- Sits on the home network; depends on the high-speed link to `cellar` for its
+  root storage.