Trigger CI on next, not only main (closes #122)

check.yml ran only on push to main and pull_request against main. Under
the current workflow every unit is a PR based on next, and next is
pushed to on each squash-merge, so neither got a CI run; the milestone
PR from next to main was the first place a broken next would surface.
Add next to both the push and pull_request branch lists so unit PRs and
next itself are checked. The workflow is otherwise unchanged: same
pinned actions/checkout sha, same script/cibuild step. README's
Entrypoints section and TODO.md's log record where CI now runs.

Model: opus-4-8

.gitea/workflows/check.yml

@@ -1,9 +1,9 @@
 name: check
 on:
   push:
-    branches: [main]
+    branches: [main, next]
   pull_request:
-    branches: [main]
+    branches: [main, next]
 jobs:
   check:
     runs-on: ubuntu-latest