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

src/shared/constants/cliTools.js

@@ -57,8 +57,13 @@ export const MITM_TOOLS = {
     color: "#FF6B00",
     description: "Kiro IDE with MITM",
     configType: "mitm",
-    mitmDomain: "q.us-east-1.amazonaws.com",
+    mitmDomain: "runtime.us-east-1.kiro.dev",
     defaultModels: [
+      // Kiro's agent/"vibe" mode sends modelId "auto" for the main turn and "simple-task"
+      // for background sub-tasks (verified via MITM request dump of generateAssistantResponse).
+      // Both need a mappable slot — otherwise getMappedModel returns null and the chat call
+      // is passed through to AWS instead of being routed to the chosen provider.
+      { id: "auto", name: "Auto (Kiro Agent)", alias: "auto" },
       { id: "claude-sonnet-5", name: "Claude Sonnet 5", alias: "claude-sonnet-5" },
       { id: "claude-sonnet-4.5", name: "Claude Sonnet 4.5", alias: "claude-sonnet-4.5" },
       { id: "claude-sonnet-4", name: "Claude Sonnet 4", alias: "claude-sonnet-4" },