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/home/stream.md

@@ -0,0 +1,19 @@
+# stream
+
+One of the two home routers. `stream` is standalone hardware; it forms a
+high-availability pair with `river` (a VM on `palace`).
+
+- **Source:** [`nixos/boxes/home/stream.nix`](../../../nixos/boxes/home/stream.nix),
+  built from [`routing-common`](../../../nixos/boxes/home/routing-common) (instance `1`)
+- **Deploy address:** `192.168.68.2`
+
+## Role
+
+- Same [`routing-common`](../../../nixos/boxes/home/routing-common) role as
+  [`river`](river.md): keepalived/VRRP, kea DHCP, radvd, DNS + blocklist, NAT and
+  the AS211024 L2 mesh link to colony.
+- Additionally pulls in `mstpd` (`routing-common/mstpd.nix`) for spanning-tree on
+  its bridged ports — `stream` is the one wired into the physical switching, so
+  it manages the L2 topology.
+
+See [river.md](river.md) for the other half of the pair.