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

`check.yml` ran only on push to `main` and on pull requests against `main`. Every unit is a PR based on `next`, and `next` is pushed on each squash-merge, so no unit PR and no push to `next` ever ran CI; a broken `next` would first surface on the milestone PR. `next` is added to both branch lists; nothing else in the workflow changes. The README Entrypoints section now says where CI runs.

Disclosure: the CI run on the PR itself fired (the proof the trigger works) but was red because the runner had no disk space left before any check step ran; the local gate was green.

Model: opus-4-8 (implementation and review)
model: claude-fable-5

.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