fix(cli): keep mini-fast primary in openai-only install catalog

OPENAI_ONLY_AGENT_OVERRIDES was rewriting explore and librarian back to
gpt-5.4 medium for OpenAI-only installs. Match the runtime primary so the
install default stays on gpt-5.4-mini-fast.

src/cli/model-fallback.test.ts

@@ -355,9 +355,9 @@ describe("generateModelConfig", () => {
       // #when generateModelConfig is called
       const result = generateModelConfig(config)
 
-      // #then explore should use native OpenAI model
-      expect(result.agents?.explore?.model).toBe("openai/gpt-5.4")
-      expect(result.agents?.explore?.variant).toBe("medium")
+      // #then explore should use native OpenAI mini-fast (primary model)
+      expect(result.agents?.explore?.model).toBe("openai/gpt-5.4-mini-fast")
+      expect(result.agents?.explore?.variant).toBeUndefined()
     })
 
     test("explore uses gpt-5-mini when only Copilot available", () => {