fix(image): align cf_model proxy schema and billing

[stackbilt-admin]

Summary

• exposes img-forge cf_model in the Stackbilder MCP image_generate schema
• maps cf_model values to effective billing/access tiers
• removes the misleading default from Gemini-only image_size and documents that it should not be sent with cf_model
• adds catalog, cost, and dispatch regression coverage

Validation

• npx vitest run test/tool-registry.test.ts test/cost-attribution.test.ts test/gateway.test.ts
• npm run typecheck

Upstream

Pair with Stackbilt-dev/img-forge PR for the actual backend model catalog expansion.