feat(pi-web-search): 统一 web_search/web_fetch 并由主模型路由搜索厂商

7 个按厂商命名的搜索工具收敛为 2 个统一工具，schema 与提示词不再暴露
provider 概念：web_search 由当前主模型按内置规则在 Tavily/Exa/Keenable
间路由（会话内 LRU 缓存，失败按可用性降级），web_fetch 固定走 Exa 并
静默降级。三家均改为纯 fetch 直连——Exa 直接对 mcp.exa.ai 发 JSON-RPC
并缓存会话，彻底移除 pi-mcp-adapter、@tavily/pi-extension 与
@keenable/pi-search 依赖。

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

pi-tool-search/README.md

@@ -32,9 +32,7 @@ Run `/tool-search-status` to inspect the current catalog source, pinned count, d
 | `ssh-remote-search` | `ssh_find`, `ssh_grep` |
 | `ssh-remote-shell` | `ssh_cd`, `ssh_bash` |
 | `code-intelligence` | CodeGraph and all LSP tools |
-| `web-tavily` | Tavily search/fetch |
-| `web-exa` | Exa search/advanced/fetch |
-| `web-keenable` | Keenable search/fetch |
+| `web-search` | `web_search`, `web_fetch` (pi-web-search unified tools) |
 | `context-execution` | `ctx_execute`, `ctx_execute_file`, `ctx_batch_execute` |
 | `context-knowledge` | Context indexing, searching, and web-doc ingestion |
 | `context-observability` | Context stats, doctor, and Insight |
@@ -55,7 +53,7 @@ Pinned tools are removed from these dynamic groups at runtime. The defaults pin
 Call `tool_search` with an exact catalog group id when possible:
 
 ```json
-{ "group": "web-tavily" }
+{ "group": "web-search" }
 ```
 
 A workflow can atomically activate up to five exact groups in one call. The complete request must fit the configured group and tool limits; otherwise no active state is changed:
@@ -114,7 +112,7 @@ Example override:
 ```json
 {
   "groupOverrides": {
-    "web-tavily": ["tavily_web_search", "tavily_web_fetch"],
+    "web-search": ["web_search", "web_fetch"],
     "lsp": ["lsp_definition", "lsp_references", "lsp_hover", "lsp_symbols", "lsp_diagnostics"]
   }
 }