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/colony/chatterbox.md

@@ -0,0 +1,19 @@
+# chatterbox
+
+The Matrix homeserver (`nul.ie`) and its chat-network bridges.
+
+- **Source:** [`shill/containers/chatterbox.nix`](../../../nixos/boxes/colony/vms/shill/containers/chatterbox.nix)
+- **Host:** NixOS container on `shill`
+
+## Role
+
+- **Matrix homeserver** for `server_name = "nul.ie"`.
+- **Bridges** to other chat networks:
+  - `heisenbridge` (IRC),
+  - `mautrix-whatsapp` (WhatsApp),
+  - `mautrix-meta` / `mautrix-messenger` (Facebook Messenger / Instagram).
+- Fronted by `middleman` (federation on `:8448`).
+
+## Networking
+
+- `internal` assignment on the `ctrs` network (alt name `chatterbox-ctr`).