prime-agent kit: models.json install step (consumes the catalog emitter) #373

Part of epic #368. Author prime-agent-install-models-json.sh: startup phase, agent user, USAI_API_KEY-gated, non-destructive (never clobber an existing ~/.prime/agent/models.json; chmod 0600). The committed models.json comes from the USAi catalog prime-agent emitter — depends on #362 (epic #357). Standalone (no usai kit) = no-op. Deps: #369, #362. Live-test: yes (compose with usai-provider + bound key → config installed; standalone → no-op).