fix: release script handles already-bumped version gracefully

Skip the version bump commit if nothing changed, so re-running
script/release for the same version doesn't fail on empty commit.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

script/release

@@ -59,7 +59,11 @@ PACKAGE_LOCK="${SCRIPT_DIR}/frontend/package-lock.json"
 echo "==> Regenerated package-lock.json"
 
 git add "$CARGO_TOML" "$CARGO_LOCK" "$PACKAGE_JSON" "$PACKAGE_LOCK"
-git commit -m "Bump version to ${VERSION}"
+if git diff --cached --quiet; then
+  echo "==> Version already at ${VERSION}, skipping commit"
+else
+  git commit -m "Bump version to ${VERSION}"
+fi
 
 if ! command -v cross >/dev/null 2>&1; then
   echo "Error: 'cross' is not installed. Run: cargo install cross"