feat(model-requirements): add k2p5, kimi-k2.5, gpt-5.4 medium to Sisyphus fallback chain

Sisyphus can now fall back through Kimi and OpenAI models when Claude
is unavailable, enabling OpenAI-only users to use Sisyphus directly
instead of being redirected to Hephaestus.

Runtime chain: claude-opus-4-6 max → k2p5 → kimi-k2.5 → gpt-5.4 medium → glm-5 → big-pickle
CLI chain: claude-opus-4-6 max → k2p5 → gpt-5.4 medium → glm-5

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

@@ -224,6 +224,10 @@ exports[`generateModelConfig single native provider uses OpenAI models when only
       "model": "openai/gpt-5.4",
       "variant": "high",
     },
+    "sisyphus": {
+      "model": "openai/gpt-5.4",
+      "variant": "medium",
+    },
   },
   "categories": {
     "deep": {
@@ -293,6 +297,10 @@ exports[`generateModelConfig single native provider uses OpenAI models with isMa
       "model": "openai/gpt-5.4",
       "variant": "high",
     },
+    "sisyphus": {
+      "model": "openai/gpt-5.4",
+      "variant": "medium",
+    },
   },
   "categories": {
     "deep": {

src/cli/config-manager.test.ts

@@ -249,8 +249,9 @@ describe("generateOmoConfig - model fallback system", () => {
     // #when generating config
     const result = generateOmoConfig(config)
 
-    // #then Sisyphus is omitted (requires all fallback providers)
-    expect((result.agents as Record<string, { model: string }>).sisyphus).toBeUndefined()
+    // #then Sisyphus resolves to gpt-5.4 medium (openai is now in sisyphus chain)
+    expect((result.agents as Record<string, { model: string; variant?: string }>).sisyphus.model).toBe("openai/gpt-5.4")
+    expect((result.agents as Record<string, { model: string; variant?: string }>).sisyphus.variant).toBe("medium")
     // #then Oracle should use native OpenAI (first fallback entry)
     expect((result.agents as Record<string, { model: string }>).oracle.model).toBe("openai/gpt-5.4")
     // #then multimodal-looker should use native OpenAI (first fallback entry is gpt-5.3-codex)

src/cli/model-fallback-requirements.ts

@@ -13,6 +13,7 @@ export const CLI_AGENT_MODEL_REQUIREMENTS: Record<string, ModelRequirement> = {
         variant: "max",
       },
       { providers: ["kimi-for-coding"], model: "k2p5" },
+      { providers: ["openai", "github-copilot", "opencode"], model: "gpt-5.4", variant: "medium" },
       { providers: ["zai-coding-plan", "opencode"], model: "glm-5" },
     ],
     requiresAnyModel: true,

src/cli/model-fallback.test.ts

@@ -396,7 +396,7 @@ describe("generateModelConfig", () => {
       expect(result.agents?.sisyphus?.model).toBe("anthropic/claude-opus-4-6")
     })
 
-    test("Sisyphus is omitted when no fallback provider is available (OpenAI not in chain)", () => {
+    test("Sisyphus resolves to gpt-5.4 medium when only OpenAI is available", () => {
       // #given
       const config = createConfig({ hasOpenAI: true })
 
@@ -404,7 +404,8 @@ describe("generateModelConfig", () => {
       const result = generateModelConfig(config)
 
       // #then
-      expect(result.agents?.sisyphus).toBeUndefined()
+      expect(result.agents?.sisyphus?.model).toBe("openai/gpt-5.4")
+      expect(result.agents?.sisyphus?.variant).toBe("medium")
     })
   })