ci: check reuse.software licensing

It's way too easy to forget to run `reuse lint` and get yourself out of
sync... I already wrote a tangled pipeline to use reuse to check this on
pull requests, it would seem good to use it here too

.tangled/workflows/reuse.yml

···
+# SPDX-FileCopyrightText: 2025 FreshlyBakedCake
+#
+# SPDX-License-Identifier: MIT
+
+when:
+  - event: ["push", "pull_request"]
+    branch: ["main"]
+
+engine: nixery
+
+dependencies:
+  nixpkgs:
+    - reuse
+
+steps:
+  - name: Check for REUSE compliance
+    command: reuse lint