fix(reviser): remove length guardrail from auto mode prompt

Auto mode delegates length entirely to normalize. Removed the
"字数护栏" block and lengthGuardrail from auto system prompt so
the reviser doesn't try to compete with normalize on length control.

Legacy modes (spot-fix, rewrite, etc. via manual CLI) retain the
length guardrail since they don't go through the normalize loop.

All 516 tests pass.

packages/core/src/agents/reviser.ts

@@ -159,7 +159,9 @@ export class ReviserAgent extends BaseAgent {
           ? `\n\nProtagonist lock: ${bookRules.protagonist.name} — ${bookRules.protagonist.personalityLock.join(", ")}. Revisions must not violate the protagonist profile.`
           : `\n\n主角人设锁定：${bookRules.protagonist.name}，${bookRules.protagonist.personalityLock.join("、")}。修改不得违反人设。`)
       : "";
-    const lengthGuardrail = options?.lengthSpec
+    // Length guardrail only used by legacy modes (manual CLI revise).
+    // Auto mode delegates length to normalize, not reviser.
+    const lengthGuardrail = mode !== "auto" && options?.lengthSpec
       ? (isEnglish
           ? "\n8. Keep the chapter word count within the target range; only allow minor deviation when fixing critical issues truly requires it"
           : "\n8. 保持章节字数在目标区间内；只有在修复关键问题确实需要时才允许轻微偏离")
@@ -229,7 +231,8 @@ export class ReviserAgent extends BaseAgent {
     const reducedControlBlock = options?.chapterIntent && options.contextPackage && options.ruleStack
       ? this.buildReducedControlBlock(options.chapterIntent, options.contextPackage, options.ruleStack)
       : "";
-    const lengthGuidanceBlock = options?.lengthSpec
+    // Length guardrail only in legacy modes — auto mode delegates length to normalize.
+    const lengthGuidanceBlock = mode !== "auto" && options?.lengthSpec
       ? `\n## 字数护栏\n目标字数：${options.lengthSpec.target}\n允许区间：${options.lengthSpec.softMin}-${options.lengthSpec.softMax}\n极限区间：${options.lengthSpec.hardMin}-${options.lengthSpec.hardMax}\n如果修正后超出允许区间，请优先压缩冗余解释、重复动作和弱信息句，不得新增支线或删掉核心事实。\n`
       : "";
     const styleGuideBlock = reducedControlBlock.length === 0