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/plugin-handlers/config-handler.test.ts

@@ -750,7 +750,7 @@ describe("Prometheus category config resolution", () => {
 
     // then
     expect(config).toBeDefined()
-    expect(config?.model).toBe("openai/gpt-5.4")
+    expect(config?.model).toBe("openai/gpt-5.5")
     expect(config?.variant).toBe("xhigh")
   })
 
@@ -810,7 +810,7 @@ describe("Prometheus category config resolution", () => {
 
     // then - falls back to DEFAULT_CATEGORIES
     expect(config).toBeDefined()
-    expect(config?.model).toBe("openai/gpt-5.4")
+    expect(config?.model).toBe("openai/gpt-5.5")
     expect(config?.variant).toBe("xhigh")
   })