Fix archive URL detection using explicit HTTP status check

Replace curl -f exit code check with explicit %{http_code} comparison
to reliably detect whether the new _mcp_ archive exists for a release.
The previous --head check gave false positives on Linux due to redirect
chain behavior, causing installs to fail with 404 for older releases.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: foxish

scripts/install.sh

@@ -65,7 +65,8 @@ _download_url() {
   # Try new naming convention (with _mcp_), fall back to old for older releases
   new_url="https://github.com/signadot/cli/releases/download/${tag}/signadot-cli_mcp_${os}_${arch}.tar.gz"
   old_url="https://github.com/signadot/cli/releases/download/${tag}/signadot-cli_${os}_${arch}.tar.gz"
-  if curl -sSLf --head "$new_url" > /dev/null 2>&1; then
+  status=$(curl -sSL --head -o /dev/null -w "%{http_code}" "$new_url" 2>/dev/null)
+  if [ "$status" = "200" ]; then
     echo "$new_url"
   else
     echo "$old_url"