feat(catalog): centralize model policy helpers

[Patel230]

Summary

• add Eyrie catalog helpers for model cost tiers, role defaults, preferred models, and provider model selection
• expose registry-ordered tier model preferences so host apps do not hardcode provider fallback lists
• add catalog policy tests covering tier, cost, and role behavior

Validation

• golangci-lint run ./catalog --timeout=5m
• go test ./catalog -run 'TestModelPolicy|TestGetPreferredProviderModel|TestModelTierAliases' -count=1
• pre-push: govulncheck, go test ./..., go vet ./...