One meter: compute credits

MemHub used to meter messages, runs, and API calls separately, which made it impossible to answer "what is this costing me?" without adding up three numbers. There is now one meter: compute credits, with an itemized spend log showing what each run consumed.

Overage is card-gated rather than surprising, Pro and Scale no longer cap members (only Free does, at 5), and the Free plan's agent-brain limit went from 3 to 15. Annual billing is two months free — the old "20%" copy was wrong.