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/waffletail.md

@@ -0,0 +1,19 @@
+# waffletail
+
+The colony Tailscale node / subnet router.
+
+- **Source:** [`shill/containers/waffletail.nix`](../../../nixos/boxes/colony/vms/shill/containers/waffletail.nix)
+- **Host:** NixOS container on `shill`
+
+## Role
+
+- Joins the Tailscale tailnet (auth key from secrets) and **advertises the colony
+  prefixes** into it, acting as the subnet router so tailnet clients can reach
+  colony services and vice-versa.
+- nftables rules SNAT/forward between `host0` and `tailscale0` for the colony
+  v4/v6 ranges. `shill` routes the Tailscale prefixes here.
+
+## Networking
+
+- `internal` assignment on the `ctrs` network (alt name `waffletail-ctr`); owns
+  the `tailscale0` interface.