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/misc/installer.md

@@ -0,0 +1,19 @@
+# installer
+
+The custom NixOS installer image used to bootstrap new boxes.
+
+- **Source:** [`nixos/installer.nix`](../../../nixos/installer.nix)
+
+## Role
+
+- Defines `nixos.systems.installer`, a minimal system whose `my.buildAs.*`
+  outputs produce installable artifacts — primarily a bootable **ISO**
+  (`isoImage`), and the same base is reused for kexec/netboot trees.
+- Build it with the devshell commands (see [`AGENTS.md`](../../../AGENTS.md#commands)):
+  - `build-iso installer`
+  - `build-kexec installer` / `build-netboot installer`
+- A released ISO is what `colony`'s VM definitions reference as install media; the
+  `update-installer` devshell command tags a release to trigger a rebuild.
+
+This is a build target rather than a deployed machine — there is no running
+`installer` host.