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/river.md

@@ -0,0 +1,25 @@
+# river
+
+One of the two home routers. `river` is a VM on `palace`; it forms a
+high-availability pair with the standalone `stream`.
+
+- **Source:** [`nixos/boxes/home/palace/vms/river.nix`](../../../nixos/boxes/home/palace/vms/river.nix),
+  built from [`routing-common`](../../../nixos/boxes/home/routing-common) (instance `0`)
+- **Host:** VM on `palace`
+- **Deploy address:** `192.168.68.1`
+
+## Role
+
+Everything in [`routing-common`](../../../nixos/boxes/home/routing-common):
+
+- **VRRP/keepalived** failover with `stream` (`keepalived.nix`) — one router is
+  master at a time, sharing virtual IPs.
+- **DHCP** via kea (`kea.nix`), **router advertisements** via radvd
+  (`radvd.nix`).
+- **DNS** (`dns.nix`) — local resolver with a blocklist
+  (`dns-blocklist.txt`) and a periodic update script.
+- **NAT / firewall** for the home LAN, with policy routing.
+- **AS211024 L2 mesh** link back to colony/`estuary` (and the other edge
+  routers), so home and colony networks interconnect.
+
+See [stream.md](stream.md) for the other half of the pair.