refactor(rectification): split runV9AgentTurn into prepare / attempt / retry / finish

Move-only (TASK-rectification-code-split-20260926 T3/T4). No behavior,
receipt, billing, Skill or scoring change.

- agent-run.ts: 1400 -> 135 lines; runV9AgentTurn 1046 -> 25 lines, no
  nested functions. It keeps the public types, the budget constants and the
  phase order: agent-run-prepare.ts (reliability, session, year re-ask, Skill
  identity, delivered guard, reserve, turn row / replay),
  agent-run-retry.ts (attempt loop), agent-run-attempt.ts (the old nested
  streamAttempt), agent-run-finish.ts (billing settle, receipts, interview,
  finalize), agent-run-support.ts (outcome types, retry classification, turn
  receipt writers), agent-run-messages.ts (buildAgentMessages /
  buildOpeningBrief, re-exported).
- One token changed with the move: the attempt passes its own
  `previousErrorCode` argument to buildAgentMessages instead of reading the
  enclosing `lastAttemptError`; the loop passes that same value (clears the
  old unused-parameter warning).
- Tests: whole-source contracts read tests/rectification-agent-run-surface.ts;
  behavioral runV9AgentTurn tests unchanged. agent-run growth caps added.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017eEAG8HD3mm8gsKXgk8uU8

frontend/tests/rectification-answer-choice.test.ts

@@ -55,6 +55,7 @@ import {
 } from "./rectification-v9-test-support.ts";
 import { rectificationChatSurface } from "./rectification-chat-surface.ts";
 import { rectificationAgentRouteSurface, readRectificationAgentRouteFile } from "./rectification-agent-route-surface.ts";
+import { rectificationAgentRunSurface } from "./rectification-agent-run-surface.ts";
 
 const ACTION_ID = "aaaaaaaa-aaaa-4aaa-8aaa-aaaaaaaaaaaa";
 const QUESTION_ID = "question-1";
@@ -1508,7 +1509,8 @@ test("the public agent route treats structured choice as a non-model command", (
 });
 
 test("rectification attempt timeout stays under the agent route budget", () => {
-  const agentRun = readFileSync(new URL("../src/lib/rectification-agentic/v9/agent-run.ts", import.meta.url), "utf8");
+  // 原值: 读 lib/rectification-agentic/v9/agent-run.ts 单文件。新值: rectificationAgentRunSurface（agent-run.ts + 拆出的 agent-run-*.ts，见 rectification-agent-run-surface.ts）。原因: TASK-rectification-code-split-20260926 只搬不改，整文件断言跟着代码走；切片断言另行改写。
+  const agentRun = rectificationAgentRunSurface;
   const budget = readFileSync(new URL("../src/lib/rectification-run-budget.ts", import.meta.url), "utf8");
   const regenerate = readFileSync(
     new URL("../src/app/api/rectification/cases/[caseId]/turns/[turnId]/regenerate/route.ts", import.meta.url),