feat(pi-overflow): 新增统一的新工具结果展示截断扩展

此前截断分散在多层且互相冲突：Pi bash 保留尾部 50KB，RTK 兜底截断又只留
开头 12000 字符（砍掉报错与完整输出路径），pi-chrome 截开头 30K，web_fetch
各 provider 不一致。新扩展统一负责"新结果给模型看多少"：
- capture.ts 最先加载暂存原文，index.ts 最后加载（system-prompt 之前）
- 超过 24K 字符时原文落盘 <agentDir>/my-pi/pi-overflow/（700/600、原子写），
  优先取 bash fullOutputPath 完整文件；模型看到 头 + 中段信号行 + 尾 的有界
  视图，行号基于落盘原文、结尾永远可见
- overflow_query：按 id 正则搜索、行区间、jsonPath 只读查询，id 校验防穿越
- 收编：RTK truncate 默认关闭、删除 pi-chrome truncateText、web_fetch 默认
  每页 20000（上限 100000）、web_search max_results 限定 1–20
- pi-compaction 遮蔽带 details.overflow.id 的结果时在占位符提示 overflow_query
- 登记 tools.jsonc / routing / 权限 allow / tool-search 常驻与分组

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pi-web-search/README.md

@@ -6,9 +6,12 @@ Unified web search and fetch for Pi. The main model only ever sees two tools —
 - `web_search(query, max_results?, site?, published_after?, published_before?, depth?)`
   routes each query to Tavily, Exa, or Keenable using the session's current main
   model (`ctx.model` + `ctx.modelRegistry.complete`), guided by per-provider
-  rules. Results include `details.provider` / `details.routedBy` for debugging.
+  rules. `max_results` defaults to 8 and is clamped to 1–20. Results include
+  `details.provider` / `details.routedBy` for debugging.
 - `web_fetch(urls, max_characters?)` always reads pages via Exa
   (`web_fetch_exa`), degrading silently to Keenable then Tavily.
+  `max_characters` defaults to 20,000 and is clamped to 1–100,000 per page;
+  oversized fetch output is further bounded by pi-overflow downstream.
 
 ## Providers (`clients/`)