Jetpack AI: site with unlimited plan shows greyed button on My Jetpack card

- greyed out Manage button on Jetpack AI card, when the site has the unlimited plan


- we need to decide what to do there since there is no Manage screen for the AI Assistant; maybe hide the button? maybe link to a new post, with label "Start"?

Start by locating the Jetpack AI card in the My Jetpack view and reproducing the greyed Manage button for a site with the unlimited plan. Review the existing AI Assistant entry point and decide whether the button should be hidden, relabeled as “Start,” or link to a new post; done means the chosen behavior is implemented and the unlimited-plan state no longer presents an unusable action.

Written by the indexing model from the issue text.