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>
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