USAi catalog (later): default-model selection in the split pipeline #365

Part of epic #357 (Later). Today model/small_model/agent.{compaction,summary,title} live OUTSIDE the generated block in opencode.jsonc and have no home in the split. Add a neutral defaults concept to the schema + per-emitter rendering (or explicitly document them as out-of-scope hand config). CAUTION: this grows the round-trip surface beyond models — design so #360 still holds. Deps: #358, #360. Live-test: no.