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/vaultwarden.md

@@ -0,0 +1,15 @@
+# vaultwarden
+
+[Vaultwarden](https://github.com/dani-garcia/vaultwarden), a Bitwarden-compatible
+password manager.
+
+- **Source:** [`shill/containers/vaultwarden.nix`](../../../nixos/boxes/colony/vms/shill/containers/vaultwarden.nix)
+- **Host:** NixOS container on `shill`
+
+## Role
+
+- Runs Vaultwarden, fronted by `middleman` and published under `nul.ie`.
+
+## Networking
+
+- `internal` assignment on the `ctrs` network (alt name `vaultwarden-ctr`).