ci: drop push:dev trigger; PR is the gate

Forgejo Actions fired both `push` and `pull_request` events for the same
commit when pushing to dev with an open PR — doubled CI runs on every PR
commit (e.g. #182 and #183 on PR #24's HEAD `95d68e3`). Drop the push
trigger entirely; PRs against main are the only quality gate that
matters in this workflow (dev → PR → main, never direct push to main).

Trade-off: direct pushes to dev with no open PR no longer get CI on
their own. Acceptable — the moment the PR opens, the pull_request event
fires and runs the same workflow.

.forgejo/workflows/test.yml

@@ -4,8 +4,10 @@ name: test
 # locally via docker-compose; they should guard with testing.Short().
 
 on:
-  push:
-    branches: [dev]
+  # PRs against main are the gate. Direct pushes to dev no longer trigger
+  # CI on their own — opening a PR fires the pull_request event and gates
+  # the merge. Avoids the duplicate push+pull_request runs we were
+  # accumulating on every PR commit.
   pull_request:
     branches: [main]