Publish the tag message as release notes

The release previously had an empty body, so GitHub showed the raw tag
message as plain text. Take the annotated tag's body (excluding subject
and signature) and pass it to gh release create so it renders as
Markdown. Fail before the slow signed builds if the tag has no body.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/build.yml

@@ -154,6 +154,21 @@ jobs:
           VERSION=${GITHUB_REF_NAME#v}
           echo "version=$VERSION" >> "$GITHUB_OUTPUT"
 
+      # Checkout leaves a lightweight tag, so re-fetch the annotated tag
+      # to read its message. The body omits the "Release vX.Y.Z" subject
+      # and the signature. Do this before the slow builds to fail early.
+      - name: Release notes from the tag
+        run: |
+          git fetch --force origin \
+              "refs/tags/$GITHUB_REF_NAME:refs/tags/$GITHUB_REF_NAME"
+          git tag -l --format='%(contents:body)' "$GITHUB_REF_NAME" \
+              >"$RUNNER_TEMP/notes.md"
+          if ! grep -q '[^[:space:]]' "$RUNNER_TEMP/notes.md"; then
+              echo "::error::tag $GITHUB_REF_NAME has no message body"
+              exit 1
+          fi
+          cat "$RUNNER_TEMP/notes.md"
+
       - name: Build signed image (x64)
         uses: docker/build-push-action@v7
         with:
@@ -216,6 +231,7 @@ jobs:
           gh release create "$GITHUB_REF_NAME" \
             --repo "$GITHUB_REPOSITORY" \
             --title "$VERSION" \
+            --notes-file "$RUNNER_TEMP/notes.md" \
             "${FLAGS[@]}" \
             w64devkit-*.7z.exe \
             source.tar