test: fix remaining stale gpt-5.4 expectations after gpt-5.5 promotion

Commit 708891dab fixed most test expectations after the gpt-5.5 model
promotion but missed 13 tests across 6 files that still expected
openai/gpt-5.4 in DEFAULT_CATEGORIES and AGENT_MODEL_REQUIREMENTS.

Updates all remaining stale expectations to openai/gpt-5.5:
- agents/utils.test.ts: atlas/metis resolution, buildAgent category,
  override.category expansion (5 tests)
- plugin-handlers/config-handler.test.ts: ultrabrain config resolution
  and fallback (2 tests)
- shared/agent-variant.test.ts: sisyphus chain variant and category
  fallback (2 tests)
- shared/model-capability-guardrails.test.ts: built-in requirement
  model ID assertion (1 test)
- tools/look-at/multimodal-fallback-chain.test.ts: multimodal-looker
  hardcoded variant metadata (1 test)
- cli/config-manager/generate-omo-config.test.ts: sisyphus model and
  fallback_models expectations (2 tests)

src/shared/agent-variant.test.ts

@@ -124,10 +124,10 @@ describe("resolveVariantForModel", () => {
     expect(variant).toBe("medium")
   })
 
-  test("returns medium for openai/gpt-5.4 in sisyphus chain", () => {
-    // #given openai/gpt-5.4 is now in sisyphus fallback chain with variant medium
+  test("returns medium for openai/gpt-5.5 in sisyphus chain", () => {
+    // #given openai/gpt-5.5 is now in sisyphus fallback chain with variant medium
     const config = {} as OhMyOpenCodeConfig
-    const model = { providerID: "openai", modelID: "gpt-5.4" }
+    const model = { providerID: "openai", modelID: "gpt-5.5" }
 
     // when
     const variant = resolveVariantForModel(config, "sisyphus", model)
@@ -179,7 +179,7 @@ describe("resolveVariantForModel", () => {
         "custom-agent": { category: "ultrabrain" },
       },
     } as OhMyOpenCodeConfig
-    const model = { providerID: "openai", modelID: "gpt-5.4" }
+    const model = { providerID: "openai", modelID: "gpt-5.5" }
 
     // when
     const variant = resolveVariantForModel(config, "custom-agent", model)

src/shared/model-capability-guardrails.test.ts

@@ -20,7 +20,7 @@ describe("model-capability-guardrails", () => {
     expect(modelIDs).toEqual([...modelIDs].sort())
     expect(new Set(modelIDs).size).toBe(modelIDs.length)
     expect(modelIDs).toContain("claude-opus-4-7")
-    expect(modelIDs).toContain("gpt-5.4")
+    expect(modelIDs).toContain("gpt-5.5")
     expect(modelIDs).toContain("kimi-k2.5")
   })