fix: add User-Agent header to crates.io API requests

crates.io's data access policy requires a descriptive User-Agent header.
Without one, the default curl/X.Y.Z agent gets 403'd, breaking the CI
"Determine core version and publication status" step.

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

Author: pRizz

scripts/check-crate-version-published.sh

@@ -18,6 +18,7 @@ http_status="$(
     --silent \
     --show-error \
     --location \
+    --user-agent "opencode-cloud-ci (https://github.com/pRizz/opencode-container)" \
     --output "${tmp_response}" \
     --write-out '%{http_code}' \
     "${api_url}"