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": {