fix(kiro): never send a top-level systemPrompt (400 REQUEST_BODY_INVALID)

kiro.dev rejects any body carrying a top-level systemPrompt with
400 REQUEST_BODY_INVALID. The translators stopped emitting the field in
v0.5.59 (the prompt travels in the first user turn via contentPrefix),
but two paths kept writing it back downstream of the translator:

- rtk/systemInject.js::injectKiroSystem() appended the RTK prompt to
  body.systemPrompt, so every kr/ model failed whenever an RTK injector
  (caveman, ponytail) was active. It now appends to the first history
  user turn's content (else currentMessage), reusing
  dedupStringAppend/hasPrompt so retries stay idempotent.
- executors/kiro.js::appendRepairInstruction() wrote the tool-call repair
  instruction to systemPrompt on the retry, turning every repair into a
  hard failure. It now appends to currentMessage.userInputMessage.content.

isKiroBody() no longer requires a string body.systemPrompt — that marker
is gone from the wire shape — and sniffs the conversation turn shape
instead, keeping the stray-conversationState guard intact. Stale comments
in both kiro translators corrected: the systemPrompt local is only a
session-replay cache key, not a wire field.

Also drops the mirror/rollback repair heuristic the injector no longer
needs: net -52 lines.

Fixes #3641, #3845, #2890, #2901, #2939, #3109, #3459, #3749

open-sse/translator/request/claude-to-kiro.js

@@ -242,9 +242,9 @@ export function claudeToKiroRequest(model, body, stream, credentials) {
     ? (credentials?.providerSpecificData?.profileArn || "")
     : (credentials?.providerSpecificData?.profileArn || resolveDefaultProfileArn(authMethod));
 
-  // Kiro CLI/KAS sends system prompt as top-level `systemPrompt`. Keep a
-  // content fallback too because the CodeWhisperer surface does not always
-  // enforce top-level systemPrompt for direct calls.
+  // The system prompt travels inside the first user turn's content (contentPrefix):
+  // the CodeWhisperer surface rejects a top-level `systemPrompt` with
+  // 400 REQUEST_BODY_INVALID, so the value below is only a replay cache key.
   const timestamp = new Date().toISOString();
   const systemPromptParts = [];
   if (thinkingBudget !== null && !usesNativeGptEffort) {

open-sse/translator/request/openai-to-kiro.js

@@ -340,9 +340,9 @@ export function openaiToKiroRequest(model, body, stream, credentials) {
 
   const timestamp = new Date().toISOString();
 
-  // Kiro CLI/KAS sends these as top-level systemPrompt. Keep a content fallback
-  // too because the CodeWhisperer surface does not always enforce top-level
-  // systemPrompt for direct calls.
+  // The system prompt travels inside the first user turn's content (contentPrefix):
+  // the CodeWhisperer surface rejects a top-level `systemPrompt` with
+  // 400 REQUEST_BODY_INVALID, so the value below is only a replay cache key.
   const systemPromptParts = [];
   if (thinkingBudget !== null && !usesNativeGptEffort) {
     systemPromptParts.push(buildThinkingSystemPrefix(thinkingBudget));