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.test.ts

@@ -98,28 +98,26 @@ describe("AGENT_MODEL_REQUIREMENTS", () => {
     expect(quaternary.model).toBe("gpt-5-nano")
   })
 
-  test("multimodal-looker has valid fallbackChain with gpt-5.3-codex as primary", () => {
+  test("multimodal-looker has valid fallbackChain with gpt-5.4 as primary", () => {
     // given - multimodal-looker agent requirement
     const multimodalLooker = AGENT_MODEL_REQUIREMENTS["multimodal-looker"]
 
     // when - accessing multimodal-looker requirement
-    // then - fallbackChain: gpt-5.3-codex -> k2p5 -> gemini-3-flash -> glm-4.6v -> gpt-5-nano
+    // then - fallbackChain: gpt-5.4 -> k2p5 -> gemini-3-flash -> glm-4.6v -> gpt-5-nano
     expect(multimodalLooker).toBeDefined()
     expect(multimodalLooker.fallbackChain).toBeArray()
     expect(multimodalLooker.fallbackChain).toHaveLength(5)
 
     const primary = multimodalLooker.fallbackChain[0]
     expect(primary.providers).toEqual(["openai", "opencode"])
-    expect(primary.model).toBe("gpt-5.3-codex")
+    expect(primary.model).toBe("gpt-5.4")
     expect(primary.variant).toBe("medium")
-
     const secondary = multimodalLooker.fallbackChain[1]
     expect(secondary.providers).toEqual(["kimi-for-coding"])
     expect(secondary.model).toBe("k2p5")
 
     const tertiary = multimodalLooker.fallbackChain[2]
     expect(tertiary.model).toBe("gemini-3-flash")
-
     const last = multimodalLooker.fallbackChain[4]
     expect(last.providers).toEqual(["openai", "github-copilot", "opencode"])
     expect(last.model).toBe("gpt-5-nano")

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" },