feat!: make web search Exa-only

BREAKING CHANGE: remove Tavily, Firecrawl, provider fallback,
and web-search-config. web_search and web_fetch now use
Exa-shaped inputs and return raw Exa-style details.

package.json

@@ -1,7 +1,7 @@
 {
   "name": "pi-web-search",
   "version": "0.1.0",
-  "description": "Pi extension package that adds web_search and web_fetch tools backed by pluggable providers such as Exa and Tavily.",
+  "description": "Pi extension package that adds Exa-backed web_search and web_fetch tools.",
   "type": "module",
   "keywords": ["pi-package"],
   "repository": {