feat(model-requirements): set multimodal-looker primary model to gpt-5.4 medium

Change multimodal-looker's primary model from gpt-5.3-codex to gpt-5.4 medium
in both runtime and CLI fallback chains.

Changes:
- Runtime chain (src/shared/model-requirements.ts): primary now gpt-5.4
- CLI chain (src/cli/model-fallback-requirements.ts): primary now gpt-5.4
- Updated test expectations in model-requirements.test.ts
- Updated config-manager.test.ts assertion
- Updated model-fallback snapshots

src/shared/model-requirements.ts

@@ -90,7 +90,7 @@ export const AGENT_MODEL_REQUIREMENTS: Record<string, ModelRequirement> = {
     fallbackChain: [
       {
         providers: ["openai", "opencode"],
-        model: "gpt-5.3-codex",
+        model: "gpt-5.4",
         variant: "medium",
       },
       { providers: ["kimi-for-coding"], model: "k2p5" },