docs: Add fleet documentation

Add a top-level `README.md` mapping the fleet 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>

AGENTS.md

@@ -91,6 +91,11 @@ Per-host configs live under `nixos/boxes/<host>` (some are single `.nix` files,
 with nested VMs/containers under e.g. `colony/vms`). Many "systems" are VMs or containers managed
 via the `vms` / `containers` modules and the `l2mesh` VXLAN module.
 
+For a human-readable map of what is actually deployed (per-machine roles, services and networking),
+see `README.md` and the per-machine docs under `docs/boxes/` (grouped `docs/boxes/colony/`,
+`docs/boxes/home/`, `docs/boxes/misc/`). Keep these in sync when adding, removing or repurposing
+a machine or service.
+
 ## Secrets
 
 age-encrypted secrets in `secrets/`, managed with **ragenix**. Each module declares