feat: migrate CI and releases from GitHub Actions to Gitea Actions

Retarget GoReleaser to publish to Gitea (gitea_urls, release.gitea,
changelog.use: gitea). Add Gitea Actions workflows for build/test,
release, and benchmarks — adapted from GitHub equivalents with macOS
jobs and SLSA provenance dropped. Old .github/workflows/ kept in place.

.gitea/workflows/release.yml

@@ -0,0 +1,62 @@
+name: Release
+
+on:
+  push:
+    tags:
+      - "v*"
+
+run-name: "Release ${{ github.ref_name }}"
+
+jobs:
+  goreleaser:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+          cache: true
+
+      - name: Run GoReleaser
+        uses: goreleaser/goreleaser-action@v6
+        with:
+          distribution: goreleaser
+          version: "~> v2"
+          args: release --clean
+        env:
+          GITEA_TOKEN: ${{ secrets.GITEA_TOKEN }}
+          GORELEASER_FORCE_TOKEN: gitea
+
+  publish-version:
+    needs: [goreleaser]
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout gh-pages
+        uses: actions/checkout@v4
+        with:
+          ref: gh-pages
+
+      - name: Update latest version
+        run: |
+          echo "${{ github.ref_name }}" > latest.txt
+
+          cat > latest.json << EOF
+          {
+            "version": "${{ github.ref_name }}",
+            "published_at": "$(date -u +%Y-%m-%dT%H:%M:%SZ)",
+            "url": "https://gitea.app.monadical.io/monadical/greywall/releases/tag/${{ github.ref_name }}"
+          }
+          EOF
+
+      - name: Commit and push to gh-pages
+        run: |
+          git config user.name "gitea-actions[bot]"
+          git config user.email "gitea-actions[bot]@noreply.gitea.app.monadical.io"
+          git add latest.txt latest.json
+          git commit -m "Update latest version to ${{ github.ref_name }}" || echo "No changes to commit"
+          git push origin gh-pages