chore: CI badge, mirror PR explainer, and the tag-pin release step (#47)

Three items #47 absorbed from #46, held back so a dead badge would not sit
beside an unresolved r-universe one. Both resolve now that v0.4.0 is tagged.

- R-CMD-check badge pointing at the GitHub mirror's workflow, where the
  4-platform matrix actually runs.

- A pull_request_target workflow explaining the mirror flow on every incoming
  PR. A PR here is landed on Gitea and syncs back, and because the merge
  preserves the contributor's commits at their original SHAs, GitHub marks the
  PR 'Merged' with nobody visibly clicking Merge. To a first-time contributor
  that reads as rejection. Say so before it happens.

  pull_request_target rather than pull_request because a fork PR's token is
  read-only under the latter -- it could not comment, which is the entire job.
  That is only safe because this never checks out or runs contributor code; the
  file says so and says not to add a checkout.

- CONTRIBUTING's release checklist now spells out that the tag goes on Gitea and
  the mirror carries it, and that r-universe does NOT pick up a release until
  packages.json's branch pin is edited. '*release' would automate it but needs a
  GitHub Release object, and the mirror pushes tags only -- so it would silently
  never update. Learned while doing this release.

CONTRIBUTING.md

@@ -101,5 +101,13 @@ but "usually" is not a release gate.
    variables set. This is the only check that catches a
    corpus-unreachable defect, and its absence is why 0.3.0 needed fixing.
 8. Bump `Version` and add a `NEWS.md` section.
-9. Tag, then update the r-universe registry pin at
-   `github.com/civilytics/civilytics.r-universe.dev`.
+9. Tag on **Gitea** (`git tag -a vX.Y.Z && git push origin vX.Y.Z`). The mirror
+   workflow carries tags to GitHub on its own — confirm the tag appears at
+   `github.com/civilytics/uscogdata/tags` before continuing.
+10. Update the r-universe registry pin at
+    `github.com/civilytics/civilytics.r-universe.dev` — edit `packages.json`'s
+    `branch` to the new tag. **r-universe will not pick up a release until this
+    is edited**: the pin is a tag, deliberately, so a mid-refactor `main` is
+    never published as a release. `"branch": "*release"` would track releases
+    automatically, but it needs a GitHub *Release* object and the mirror pushes
+    tags only — so it would silently never update.