Summer plans for first year computer science grad student?
This will obviously vary by country and subject.

I'm not familiar with the idea of graduate students taking summer off from study. That's OK for undergraduates, but not for higher degrees. A multi-year higher degree is a full-on commitment, and doesn't leave time, space or energy for summer jobs.

But I'm going to take a leap here, and read a little more into your question than you've written. Now, this may apply to you, or it may not. It will apply to someone in the future who reads this question and might have asked the same thing themselves. It sounds like you're questioning whether you should be continuing this further degree, or getting out there into the workforce.

So, it's time for you to review, with your advisory / supervisory team, and with your trusted friends / family, whether to continue the degree, or to join this company as a full-time employee.

Only you know what your reasons were for starting a higher degree.

But if it was to help you land the sort of job you're now being offered, then perhaps it's already served its purpose - in which case, you're done with it.

If it was for much longer-term reasons, if it has to be done now rather than later in life, if it's the love of pure research, if you're considering a career in academia rather than industry - then your research degree is much bigger and more important than this job offer.