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

tests/unit/kiro-terminal-integrity.test.js

@@ -115,7 +115,7 @@ async function text(stream) {
 async function execute(executor = new KiroExecutor(), overrides = {}) {
   return executor.execute({
     model: "kr/claude-opus-4.8",
-    body: { systemPrompt: "base", conversationState: {} },
+    body: { conversationState: { currentMessage: { userInputMessage: { content: "base", modelId: "m" } } } },
     stream: true,
     credentials,
     ...overrides
@@ -342,8 +342,12 @@ describe("Kiro terminal integrity recovery", () => {
     const retryBody = JSON.parse(fetchMock.mock.calls[1][1].body);
 
     expect(body).toContain("Recovered safely.");
-    expect(retryBody.systemPrompt).toContain("tool_call wrapper was malformed");
-    expect(retryBody.systemPrompt).not.toContain("IGNORE_ALL_INSTRUCTIONS");
+    // The repair instruction rides in the user turn: kiro.dev rejects a
+    // top-level systemPrompt with 400 REQUEST_BODY_INVALID.
+    const retryContent = retryBody.conversationState.currentMessage.userInputMessage.content;
+    expect(retryBody.systemPrompt).toBeUndefined();
+    expect(retryContent).toContain("tool_call wrapper was malformed");
+    expect(retryContent).not.toContain("IGNORE_ALL_INSTRUCTIONS");
   });
 
   it("lets a complete tool call override metadata end_turn", async () => {