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/__snapshots__/model-fallback.test.ts.snap

@@ -206,16 +206,14 @@ exports[`generateModelConfig single native provider uses OpenAI models when only
       "variant": "medium",
     },
     "explore": {
-      "model": "openai/gpt-5.4",
-      "variant": "medium",
+      "model": "openai/gpt-5.4-mini-fast",
     },
     "hephaestus": {
       "model": "openai/gpt-5.4",
       "variant": "medium",
     },
     "librarian": {
-      "model": "openai/gpt-5.4",
-      "variant": "medium",
+      "model": "openai/gpt-5.4-mini-fast",
     },
     "metis": {
       "model": "openai/gpt-5.4",
@@ -296,16 +294,14 @@ exports[`generateModelConfig single native provider uses OpenAI models with isMa
       "variant": "medium",
     },
     "explore": {
-      "model": "openai/gpt-5.4",
-      "variant": "medium",
+      "model": "openai/gpt-5.4-mini-fast",
     },
     "hephaestus": {
       "model": "openai/gpt-5.4",
       "variant": "medium",
     },
     "librarian": {
-      "model": "openai/gpt-5.4",
-      "variant": "medium",
+      "model": "openai/gpt-5.4-mini-fast",
     },
     "metis": {
       "model": "openai/gpt-5.4",

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", () => {

src/cli/openai-only-model-catalog.test.ts

@@ -28,8 +28,8 @@ describe("generateModelConfig OpenAI-only model catalog", () => {
     const result = generateModelConfig(config)
 
     // #then
-    expect(result.agents?.explore).toEqual({ model: "openai/gpt-5.4", variant: "medium" })
-    expect(result.agents?.librarian).toEqual({ model: "openai/gpt-5.4", variant: "medium" })
+    expect(result.agents?.explore).toEqual({ model: "openai/gpt-5.4-mini-fast" })
+    expect(result.agents?.librarian).toEqual({ model: "openai/gpt-5.4-mini-fast" })
   })
 
   test("fills remaining OpenAI-only category gaps with OpenAI models", () => {

src/cli/openai-only-model-catalog.ts

@@ -1,8 +1,8 @@
 import type { AgentConfig, CategoryConfig, GeneratedOmoConfig, ProviderAvailability } from "./model-fallback-types"
 
 const OPENAI_ONLY_AGENT_OVERRIDES: Record<string, AgentConfig> = {
-  explore: { model: "openai/gpt-5.4", variant: "medium" },
-  librarian: { model: "openai/gpt-5.4", variant: "medium" },
+  explore: { model: "openai/gpt-5.4-mini-fast" },
+  librarian: { model: "openai/gpt-5.4-mini-fast" },
 }
 
 const OPENAI_ONLY_CATEGORY_OVERRIDES: Record<string, CategoryConfig> = {