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/jackflix.md

@@ -0,0 +1,25 @@
+# jackflix
+
+The media stack — acquisition, library and streaming.
+
+- **Source:** [`shill/containers/jackflix/`](../../../nixos/boxes/colony/vms/shill/containers/jackflix)
+  (`default.nix`, `networking.nix`)
+- **Host:** NixOS container on `shill`
+
+## Role
+
+- **Streaming:** Jellyfin.
+- **Acquisition (*arr stack):** Transmission, Jackett, FlareSolverr, Radarr,
+  Sonarr, and Jellyseerr (`seerr`) for requests.
+- **Photos:** PhotoPrism (`photos.nul.ie`).
+- **File sharing:** copyparty (`:3923`) serving public + private media volumes.
+- Media lives on the shared `/mnt/media` volume (bind-mounted read-write from
+  `shill`). Downloaders bind to a VPN interface
+  ([`networking.nix`](../../../nixos/boxes/colony/vms/shill/containers/jackflix/networking.nix)),
+  so torrent traffic only flows while `systemd-networkd-wait-online@vpn` is up.
+- A shared `media` group (gid 2000) gives the apps coordinated access.
+
+## Networking
+
+- `internal` assignment on the `ctrs` network (alt name `jackflix-ctr`), plus its
+  own VPN interface for the download clients.