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/plugin/event.model-fallback.test.ts

@@ -334,8 +334,8 @@ describe("createEventHandler - model fallback", () => {
 
     //#then - second fallback entry applied (chain advanced)
     expect(second.message["model"]).toEqual({
-      providerID: "zai-coding-plan",
-      modelID: "glm-5",
+      providerID: "kimi-for-coding",
+      modelID: "k2p5",
     })
     expect(second.message["variant"]).toBeUndefined()
     expect(abortCalls).toEqual([sessionID, sessionID])