Host the action on Gitea

Workflows move to .gitea/workflows, which Gitea prefers over
.github/workflows when both exist.

GitHub Actions can only resolve `uses:` against github.com, so the
README now spells out both forms: the full Gitea URL, and the
checkout-then-`uses: ./` fallback for a GitHub workflow that has no
mirror of this repository.

.gitea/workflows/release.yml

@@ -0,0 +1,36 @@
+name: release
+
+on:
+  push:
+    tags: ["v*.*.*"]
+
+permissions:
+  contents: write
+
+jobs:
+  # Workflows pin `@v1`, so the major tag has to follow each release.
+  major-tag:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+          # Needed to push the moved tag back; checkout leaves it in the git
+          # config for the later push step.
+          token: ${{ secrets.GITEA_TOKEN }}
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: "20"
+
+      - name: Test before moving the tag
+        run: node --test
+
+      - name: Move the major version tag
+        run: |
+          major="${GITHUB_REF_NAME%%.*}"
+          git config user.name "gitea-actions"
+          git config user.email "gitea-actions@noreply.localhost"
+          git tag -f "$major" "$GITHUB_REF_NAME"
+          git push origin "refs/tags/$major" --force
+          echo "Moved $major to $GITHUB_REF_NAME"