fix: use cmd /c on Windows for shell command execution

The exec tool was hardcoded to use 'sh -c' which doesn't exist on Windows,
causing all tool calls to fail silently in gateway mode.

pkg/tools/shell.go

@@ -8,6 +8,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"regexp"
+	"runtime"
 	"strings"
 	"time"
 )
@@ -91,7 +92,12 @@ func (t *ExecTool) Execute(ctx context.Context, args map[string]interface{}) (st
 	cmdCtx, cancel := context.WithTimeout(ctx, t.timeout)
 	defer cancel()
 
-	cmd := exec.CommandContext(cmdCtx, "sh", "-c", command)
+	var cmd *exec.Cmd
+	if runtime.GOOS == "windows" {
+		cmd = exec.CommandContext(cmdCtx, "cmd", "/c", command)
+	} else {
+		cmd = exec.CommandContext(cmdCtx, "sh", "-c", command)
+	}
 	if cwd != "" {
 		cmd.Dir = cwd
 	}