feat: make gpt-5.3-codex medium the primary model for multimodal-looker

GPT-5.3 Codex has strong multimodal capabilities. Promote it to first
candidate in multimodal-looker fallback chain, with gemini-3-flash
following (matching the ULW pattern of gpt-5.3-codex -> gemini).

src/cli/model-fallback-requirements.ts

@@ -44,6 +44,7 @@ export const CLI_AGENT_MODEL_REQUIREMENTS: Record<string, ModelRequirement> = {
   },
   "multimodal-looker": {
     fallbackChain: [
+      { providers: ["openai", "opencode"], model: "gpt-5.3-codex", variant: "medium" },
       { providers: ["google", "github-copilot", "opencode"], model: "gemini-3-flash" },
       { providers: ["openai", "github-copilot", "opencode"], model: "gpt-5.2" },
       { providers: ["zai-coding-plan"], model: "glm-4.6v" },