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/shared/agent-variant.test.ts

@@ -124,8 +124,8 @@ describe("resolveVariantForModel", () => {
     expect(variant).toBe("medium")
   })
 
-  test("returns undefined for provider not in sisyphus chain", () => {
-    // #given openai is not in sisyphus fallback chain anymore
+  test("returns medium for openai/gpt-5.4 in sisyphus chain", () => {
+    // #given openai/gpt-5.4 is now in sisyphus fallback chain with variant medium
     const config = {} as OhMyOpenCodeConfig
     const model = { providerID: "openai", modelID: "gpt-5.4" }
 
@@ -133,7 +133,7 @@ describe("resolveVariantForModel", () => {
     const variant = resolveVariantForModel(config, "sisyphus", model)
 
     // then
-    expect(variant).toBeUndefined()
+    expect(variant).toBe("medium")
   })
 
   test("returns undefined for provider not in chain", () => {

src/shared/model-requirements.test.ts

@@ -23,15 +23,15 @@ describe("AGENT_MODEL_REQUIREMENTS", () => {
     expect(primary.variant).toBe("high")
   })
 
-  test("sisyphus has claude-opus-4-6 as primary and requiresAnyModel", () => {
+  test("sisyphus has claude-opus-4-6 as primary with k2p5, kimi-k2.5, gpt-5.4 medium fallbacks", () => {
     // #given - sisyphus agent requirement
     const sisyphus = AGENT_MODEL_REQUIREMENTS["sisyphus"]
 
     // #when - accessing Sisyphus requirement
-    // #then - fallbackChain has claude-opus-4-6 first, big-pickle last
+    // #then - fallbackChain has 6 entries with correct ordering
     expect(sisyphus).toBeDefined()
     expect(sisyphus.fallbackChain).toBeArray()
-    expect(sisyphus.fallbackChain).toHaveLength(3)
+    expect(sisyphus.fallbackChain).toHaveLength(6)
     expect(sisyphus.requiresAnyModel).toBe(true)
 
     const primary = sisyphus.fallbackChain[0]
@@ -39,7 +39,19 @@ describe("AGENT_MODEL_REQUIREMENTS", () => {
     expect(primary.model).toBe("claude-opus-4-6")
     expect(primary.variant).toBe("max")
 
-    const last = sisyphus.fallbackChain[2]
+    const second = sisyphus.fallbackChain[1]
+    expect(second.providers).toEqual(["kimi-for-coding"])
+    expect(second.model).toBe("k2p5")
+
+    const third = sisyphus.fallbackChain[2]
+    expect(third.model).toBe("kimi-k2.5")
+
+    const fourth = sisyphus.fallbackChain[3]
+    expect(fourth.providers).toContain("openai")
+    expect(fourth.model).toBe("gpt-5.4")
+    expect(fourth.variant).toBe("medium")
+
+    const last = sisyphus.fallbackChain[5]
     expect(last.providers[0]).toBe("opencode")
     expect(last.model).toBe("big-pickle")
   })

src/shared/model-requirements.ts

@@ -20,6 +20,19 @@ export const AGENT_MODEL_REQUIREMENTS: Record<string, ModelRequirement> = {
         model: "claude-opus-4-6",
         variant: "max",
       },
+      { providers: ["kimi-for-coding"], model: "k2p5" },
+      {
+        providers: [
+          "opencode",
+          "moonshotai",
+          "moonshotai-cn",
+          "firmware",
+          "ollama-cloud",
+          "aihubmix",
+        ],
+        model: "kimi-k2.5",
+      },
+      { providers: ["openai", "github-copilot", "opencode"], model: "gpt-5.4", variant: "medium" },
       { providers: ["zai-coding-plan", "opencode"], model: "glm-5" },
       { providers: ["opencode"], model: "big-pickle" },
     ],