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/colony-psql.md

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+# colony-psql
+
+The shared PostgreSQL instance for colony. Several other containers
+(`middleman`, `chatterbox`, `toot`, `git`, …) connect here rather than each
+running their own database.
+
+- **Source:** [`shill/containers/colony-psql.nix`](../../../nixos/boxes/colony/vms/shill/containers/colony-psql.nix)
+- **Host:** NixOS container on `shill`
+
+## Role
+
+- **PostgreSQL 14** serving the other colony services over the `ctrs` network.
+  Consumers wait for it to be ready via the `systemdAwaitPostgres` helper.
+- netdata for monitoring.
+
+## Networking
+
+- `internal` assignment on the `ctrs` network (alt name `colony-psql-ctr`).