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/kelder.md

@@ -0,0 +1,24 @@
+# kelder
+
+A host at a remote site ("kelder" = cellar/basement), linked back to the rest of
+the other boxes over WireGuard. It is itself a **NixOS container host**.
+
+- **Source:** [`nixos/boxes/kelder/`](../../../nixos/boxes/kelder)
+  (`default.nix`, `boot.nix`, `containers/`, `plymouth/`)
+- **Domain:** `hentai.engineer`
+
+## Role
+
+- **Site uplink:** connects to colony's `estuary` over **WireGuard**
+  (`kelder` peer; see [estuary.md](../colony/estuary.md)), so the remote site is
+  reachable through the colony edge. A periodic `dns_update.py` keeps DNS current.
+- **Container host:** runs NixOS containers via `my.containers.instances`
+  (`acquisition`, `spoder`).
+- Custom boot/splash (`boot.nix`, Plymouth theme in `plymouth/`).
+
+## Containers
+
+| Container | Role | Docs |
+| --- | --- | --- |
+| `kelder-acquisition` | Media stack (Jellyfin + *arr + Transmission) | [kelder-acquisition.md](kelder-acquisition.md) |
+| `kelder-spoder` | nginx web host | [kelder-spoder.md](kelder-spoder.md) |