refactor(shell): interpret zero timeout as unlimited execution

Replace unconditional WithTimeout usage with conditional context creation
based on timeout configuration. Zero values now bypass timeout enforcement,
using WithCancel for graceful cancellation while preserving existing timeout
behavior for positive values. Simplifies CronTool initialization by removing
unnecessary conditional timeout assignment.

pkg/tools/shell.go

@@ -89,7 +89,14 @@ func (t *ExecTool) Execute(ctx context.Context, args map[string]interface{}) *To
 		return ErrorResult(guardError)
 	}
 
-	cmdCtx, cancel := context.WithTimeout(ctx, t.timeout)
+	// timeout == 0 means no timeout
+	var cmdCtx context.Context
+	var cancel context.CancelFunc
+	if t.timeout > 0 {
+		cmdCtx, cancel = context.WithTimeout(ctx, t.timeout)
+	} else {
+		cmdCtx, cancel = context.WithCancel(ctx)
+	}
 	defer cancel()
 
 	var cmd *exec.Cmd