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/object.md

@@ -0,0 +1,24 @@
+# object
+
+Object storage and Nix binary cache, plus a couple of small self-hosted web
+apps.
+
+- **Source:** [`shill/containers/object.nix`](../../../nixos/boxes/colony/vms/shill/containers/object.nix)
+- **Host:** NixOS container on `shill`
+
+## Role
+
+- **MinIO** — S3-compatible object storage (`s3.nul.ie` / `*.s3.nul.ie`). Backs
+  several other services (Gitea LFS/artifacts, social-media uploads, …). Stored on the
+  `/mnt/minio` volume (XFS, bind-mounted from `shill`).
+- **Harmonia** — serves the Nix binary cache for all the boxes (`nix-cache.nul.ie`), backed
+  by the `/mnt/nix-cache` volume.
+- **atticd** — an alternative Nix cache (stores into MinIO/S3). **Currently
+  disabled** — present in the config but not running.
+- **HedgeDoc** — collaborative markdown notes.
+- **wastebin** — pastebin.
+
+## Networking
+
+- `internal` assignment on the `ctrs` network (alt name `object-ctr`).
+- `/mnt/minio` and `/mnt/nix-cache` bind-mounted read-write from `shill`.