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/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" },
     ],