Design AI governance as a decision and evidence service

[simaba]

Why

The repository had useful lifecycle folders but still looked like a polished outline: forum names, risk tiers, generic controls, and approval-rate metrics without a rigorous operating-model design.

Changes

• adds an operating-model design guide built from recurring decisions, authority, evidence, service contracts, and follow-through
• redesigns the operating-model template around decision inventory, proportional paths, forum charters, ownership vs assurance, exceptions, and failure-demand metrics
• removes pass-rate style governance metrics and simplistic tier-only routing
• rewrites the README around decision service design and real-case improvement

Documentation only.