fix(model-fallback): add gpt-5-nano to multimodal-looker chain, remove librarian hardcoding

- BUG-7: Add gpt-5-nano as final fallback in multimodal-looker model requirements
- BUG-14: Remove hardcoded LIBRARIAN_MODEL, let librarian resolve through normal fallback chain
- Update snapshots and tests to reflect new fallback behavior

src/shared/model-requirements.ts

@@ -120,6 +120,7 @@ export const AGENT_MODEL_REQUIREMENTS: Record<string, ModelRequirement> = {
       fb("quotio", "claude-opus-4-6-thinking"),
       fb("quotio", "claude-sonnet-4-5-thinking"),
       fb("quotio", "claude-haiku-4-5"),
+      fb("quotio", "gpt-5-nano"),
     ],
   },
   prometheus: {