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/tools/look-at/multimodal-fallback-chain.test.ts

@@ -34,7 +34,7 @@ describe("buildMultimodalLookerFallbackChain", () => {
   it("preserves hardcoded variant metadata for cache-derived entries", async () => {
     // given
     const { buildMultimodalLookerFallbackChain } = await import("./multimodal-fallback-chain")
-    const visionCapableModels = [{ providerID: "openai", modelID: "gpt-5.4" }]
+    const visionCapableModels = [{ providerID: "openai", modelID: "gpt-5.5" }]
 
     // when
     const result = buildMultimodalLookerFallbackChain(visionCapableModels)
@@ -42,7 +42,7 @@ describe("buildMultimodalLookerFallbackChain", () => {
     // then
     expect(result[0]).toEqual({
       providers: ["openai"],
-      model: "gpt-5.4",
+      model: "gpt-5.5",
       variant: "medium",
     })
   })