Fix/plan paste fixes (#1238)

* fix: 降低 paste 检测阈值，修复非 bracketed-paste 终端粘贴文本损坏

非 bracketed-paste 终端下，短粘贴（<800 chars）的 stdin chunk 作为独立
keystroke 走 useTextInput.onInput 路径，闭包中 cursor 未刷新导致多次插入
竞态。现将 ≥3 字符的非特殊键输入纳入 paste 累积模式，绕过逐 chunk 处理。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* @
fix: Plan模式三处缺陷修复 — ExploreAgent可用性 + 弹窗一致性 + 方案文件保护

1. areExplorePlanAgentsEnabled()移除GrowthBook A/B实验依赖(tengu_amber_stoat)，
   始终返回true，确保Explore/Plan agent在BUILTIN_EXPLORE_PLAN_AGENTS开启时始终可用

2. ExitPlanMode clear-context路径补setNeedsPlanModeExitAttachment(true)，
   确保清除上下文退出Plan模式后生成plan_mode_exit附件

3. Plan mode full/sparse指令强化Plan文件读取要求：
   "can read" -> "MUST use FileRead to read first before any changes"，
   新增"do NOT overwrite"禁止覆盖，Phase 1指令强化并行Explore Agent引导

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
@

---------

Co-authored-by: psj88520 <qq18243133@gmail.com>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

packages/builtin-tools/src/tools/AgentTool/builtInAgents.ts

@@ -12,9 +12,7 @@ import type { AgentDefinition } from './loadAgentsDir.js'
 
 export function areExplorePlanAgentsEnabled(): boolean {
   if (feature('BUILTIN_EXPLORE_PLAN_AGENTS')) {
-    // 3P default: true — Bedrock/Vertex keep agents enabled (matches pre-experiment
-    // external behavior). A/B test treatment sets false to measure impact of removal.
-    return getFeatureValue_CACHED_MAY_BE_STALE('tengu_amber_stoat', true)
+    return true
   }
   return false
 }