refactor(beszel): switch to universal token via secrets/common.yaml

Replace per-host host-token.nix imports with a single beszel-token secret
in secrets/common.yaml, wired once in enable-agent.nix. Host files now only
need services.beszel.agent.environment.KEY — no imports block required.

Delete modules/beszel/host-token.nix (no longer referenced anywhere).

Action needed: run `sops secrets/common.yaml` and add `beszel-token: <value>`
from the beszel hub UI before deploying.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -21,9 +21,8 @@ machines when deployed.
   `modules/installer/common.nix` (the auto-installer's own root/nixos login —
   a deliberate, documented choice, see `docs/auto-installer.md`, not
   accidental tech debt) and **SSH public keys** in `variables.nix`
-  (`vars.adminSshKey`, `vars.remoteBuilderAuthorizedKeys`) plus a couple of
-  per-host `KEY` values for beszel-agent auth (`hosts/server/host.nix`,
-  `hosts/nix-cache/host.nix`). Don't use the installer's hardcoded hash as a
+  (`vars.adminSshKey`, `vars.remoteBuilderAuthorizedKeys`) plus per-host
+  `KEY` values for beszel-agent auth (see `docs/beszel.md`). Don't use the installer's hardcoded hash as a
   template for a *real* host — every other host uses sops-nix
   (`hashedPasswordFile`, see "Security Notes" in `README.md`). Flag any *new*
   secret-like string you encounter instead of committing it.
@@ -375,9 +374,8 @@ removing a host.
 
 - `hosts/<name>/host.nix` — per-machine identity **only**: hostname, hostId,
   per-machine secrets, `system.stateVersion`. These files carry no `imports`
-  of their own beyond narrow parameterized helpers (see
-  `modules/beszel/host-token.nix` below) — all shared behavior comes from the
-  platform/build-type modules composed in `flake.nix`, not from the host file.
+  of their own — all shared behavior comes from the platform/build-type modules
+  composed in `flake.nix`, not from the host file.
 - `modules/platforms/{linode,proxmox,lxc,baremetal}.nix` — platform-specific
   config: boot method, guest tooling, and the hardware config, imported
   directly by the platform module itself — **not** wired in from
@@ -434,10 +432,10 @@ removing a host.
   substituter + SSH remote-builder wiring; see `docs/nix-cache.md` for the
   full design (per-host local stores, no shared `/nix/store`, and how the
   `nixremote` signing/SSH keys fit together).
-- `modules/beszel/host-token.nix` — parameterized helper module
-  (`{ name, sopsFile }`) that wires a host's beszel-agent sops secret/template
-  and `environmentFile`; used by `hosts/server/host.nix` and
-  `hosts/nix-cache/host.nix` to avoid duplicating that boilerplate.
+- `modules/beszel/enable-agent.nix` — enables beszel-agent, sets `HUB_URL`,
+  fixes the upstream `StateDirectory` bug, and wires the universal
+  `beszel-token` sops secret (from `secrets/common.yaml`) into the agent's
+  `environmentFile`; see `docs/beszel.md` for the full setup guide.
 - `modules/tailscale/`, `modules/docker/`, `modules/networking/`,
   `modules/traefik/`, `modules/tor/`, `modules/services/*` — single-purpose,
   single-host