fix(tools): add proxy support to web_fetch HTTP transport

The web_fetch tool's custom http.Transport was missing the Proxy field,
causing it to ignore HTTP_PROXY/HTTPS_PROXY environment variables.
Unlike a nil Transport (which defaults to http.DefaultTransport with
ProxyFromEnvironment), a manually constructed Transport defaults Proxy
to nil (no proxy). Add Proxy: http.ProxyFromEnvironment to restore
proxy support for users behind corporate proxies or in censored networks.

Closes #413

pkg/tools/web.go

@@ -413,6 +413,7 @@ func (t *WebFetchTool) Execute(ctx context.Context, args map[string]interface{})
 	client := &http.Client{
 		Timeout: 60 * time.Second,
 		Transport: &http.Transport{
+			Proxy:               http.ProxyFromEnvironment,
 			MaxIdleConns:        10,
 			IdleConnTimeout:     30 * time.Second,
 			DisableCompression:  false,