Fix and clarify workflow logic, add branch versioning

- `pr.yml` should've been deleted a long time ago. Its functionality
  was already elsewhere, and that's since been folded into `test.yml`.
- I added automatic `release-v*` and `main-v*` versioning.
- Renamed `merge.yml` to `push-release.yml` and `push-main.yml` to
  `pr-create-release.yml` so it's clearer what they're actually
  listening to. I also modified their display names.

.github/workflows/pr-create-release.yml

@@ -0,0 +1,11 @@
+name: Warn on opening a PR to `release`
+on:
+  pull_request_target:
+    types: [opened]
+    branches: [release]
+permissions:
+  issues: write
+jobs:
+  comment:
+    uses: MithrilJS/infra/.github/workflows/reject-pr.yml@main
+    secrets: inherit