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/unifi.md

@@ -0,0 +1,23 @@
+# unifi
+
+The UniFi network controller.
+
+- **Source:** [`sfh/containers/unifi.nix`](../../../nixos/boxes/home/palace/vms/sfh/containers/unifi.nix)
+- **Host:** NixOS container on `sfh`
+
+## Status
+
+> **Currently disabled.** The system is still defined (`nixos.systems.unifi`),
+> but its import is commented out in
+> [`sfh/containers/default.nix`](../../../nixos/boxes/home/palace/vms/sfh/containers/default.nix),
+> so it is not deployed as a container right now. Re-enable by uncommenting
+> `./unifi.nix` there.
+
+## Role
+
+- Runs the UniFi controller (`services.unifi`) to manage the home UniFi network
+  gear.
+
+## Networking
+
+- `internal` assignment (alt name `unifi-ctr`).