feat: Implement audience-only secret badge for Twenty Sprouts game

feat: implement Archive feature for past session management

feat(observability): instrument models, conductor, memory, ledger (Units 2-6)

feat(core): add resilient loop and shared blackboard

feat: mem0 as a layered, derived semantic memory index

feat: phase 2 architecture — manifest system, structured output, salience memory, SQLite ledger, observer

feat: phase 1 — memory layer, governor, second scenario, 70 passing tests