fix(kiro): intercept chat via x-amz-target and prepend initial-response frame

Kiro IDE 1.0.228+ moved GenerateAssistantResponse from path
/generateAssistantResponse to POST / + x-amz-target header, so chat turns
bypassed MITM. The SmithyMessageDecoderStream also now requires an
initial-response frame at stream start, and agent/vibe mode sends
modelId "auto" which had no mappable slot.

- Add isChatRequest() header-based match for kiro in mitm/config.js
- Add buildInitialResponseFrame/withInitialFrame to emit the mandatory
  initial-response once per stream (kiro.js)
- Add "auto" model slot and update mitmDomain to runtime.us-east-1.kiro.dev

tests/unit/kiro-model-slots.test.js

@@ -14,6 +14,13 @@ describe("Kiro MITM model slots", () => {
     expect(Array.isArray(kiro.defaultModels)).toBe(true);
   });
 
+  it("offers a mappable slot for the agent default model id 'auto'", () => {
+    // اسلات auto برای vibe mode لازمه — وگرنه درخواست میره AWS
+    const auto = kiro.defaultModels.find((m) => m.id === "auto");
+    expect(auto).toBeTruthy();
+    expect(auto.alias).toBe("auto");
+  });
+
   it("offers a mappable slot for Claude Sonnet 5", () => {
     const sonnet5 = kiro.defaultModels.find((m) => m.id === "claude-sonnet-5");
     expect(sonnet5).toBeTruthy();