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/gam.md

@@ -0,0 +1,17 @@
+# gam
+
+A game-server container (the lightweight counterpart to the OCI game servers on
+`whale2`).
+
+- **Source:** [`shill/containers/gam.nix`](../../../nixos/boxes/colony/vms/shill/containers/gam.nix)
+- **Host:** NixOS container on `shill`
+
+## Role
+
+- Hosts game servers run directly as NixOS services — currently **Terraria**
+  (config/world from secrets). Exposed to the internet via `estuary`'s port
+  forwards (`:7777`).
+
+## Networking
+
+- `internal` assignment on the `ctrs` network (alt name `gam-ctr`).