Better Monster Examine v1.2.1

Fixes plugin-hub packaging. The plugin now injects the client's Gson instead of constructing its own — the hub rejects fresh Gson instances as a terminally deprecated API, which is why the v1.2 hub update failed to package. No user-facing behaviour change otherwise.

Adds CI that runs the real plugin-hub packager verification on every PR and as a release gate, so this class of failure is caught before release.