Only workspace members can start or steer a run

This is a tightening. A Sinatra label applied by someone outside the workspace, or by a bot, used to start a run that then stalled asking for payment; it now stops straight away with a message saying only invited members can run tasks. Comments were gated on GitHub organization membership rather than Sinatra membership, so an org member you never invited could steer a run, and anyone able to reply on a Linear thread could do the same.

The check is now membership in the Sinatra workspace: on GitHub we match the commenter to the GitHub account they sign in to Sinatra with, and on Linear we match the replier's linked Linear account. That also means an outside collaborator you have invited can now drive a run from a comment. Someone with a Sinatra account who is not a member gets an access request filed for the workspace owners.