Nowhere in any canon material is ambiguity or uncertainty even hinted at. Code Geass was in no shape or form meant to be open-ended, and all official materials confirm that.
Quotes from the interviews, articles, and the anime relevant to the topic. Quotes of particular interest are highlighted:
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Ichiro Okouchi, 'Continue' vol 42, (October 2008). "Lelouch dead!? Was it a good one or a bad one!?"
" -----Still, isn't it possible that defeating the wise ruler Schneizel, the person who was supposed to have brought order to the world, might lead to some [viewers] interpreting it as a Bad End?
Okouchi: That's true. There are probably a lot of people who think of it as a Bad End, a tragedy, considering the protagonist's, Lelouch's end as well. However, Lelouch says in the first episode: "Only those prepared to be shot are allowed to pull the trigger themselves." If you were to think of that as his pride, then I think his getting shot (killed) in the end was a logical end. Of course, I understand that not all of the viewers will accept this ending. There were people who wanted a happier ending, after all.
-----Was there a dispute among the staff members regarding the ending?
Okouchi: No. It was decided fairly naturally. During the "Code Geass" script meetings, there are many cases in which there were a number of disputes, but there were barely any when it came to the scripts for (the previous series's) episode 25 and the final episode. I think everyone felt the same when it came to the end of the character that is Lelouch.
-----Why were you so bold as to choose this ending when the viewers might see it as a Bad End?
Okouchi: Bold... yes, we were so bold as to chose this ending. Perhaps the show that is "Code Geass" ending up this way was decided the moment Director Taniguchi and I teamed up. I suppose you can call it our sense of aesthetics, or perhaps a part of our psychological makeup.
...
------I see. So Lelouch's decision was also your, Mr. Okouchi's, and Director Taniguchi's decision.
Okouchi: Which is why I think of both our and Lelouch's decision as Happy Ends. I believe that there will be better things in the tomorrow awaiting Nunnally, Kallen and the rest who have been left behind. And surely Lelouch, who was able to make this into a reality, can only be happy [about this].
...
-----Some unresolved mysteries still remain.
Okouchi: From the very beginning, [I/we] never planned on explaining everything. In fact, if you ask me, I think we might have overdone the explanations. While it's undeniable that Lelouch's story has ended with a full stop, the other characters' stories are still on-going, and it's not like the world [of Code Geass] itself has come to an end either. [I/we] didn't want to end it by closing it up for good."
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Animage Nov 2008, cast comments
Floating text: "C.C. - Her wish was to die as a human, but after spending time with Lelouch, C.C. also wished for tomorrow. She made up her mind to ensure the tomorrow of the world that Lelouch had left."
Animage: The ending's outcome turned out as such, but do you think Lelouch was happy?
Okouchi: I think that this (work) was not a bad end, but a good end. This already appeared in Lelouch and C.C.'s conversation in Stage 7 of S1, but people who are 'just' living their lives are uninteresting. Lelouch saved his beloved sister and made the kind world that he envisioned, a reality. His life was not in vain. That's why, Lelouch was smiling in the end."
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C.C.'s profile on the official GeassNet (mobile) website
"..Knowing that Lelouch does not hate her for giving him the Geass, she is now able to show her true feelings. With the realization of "Zero Requiem", her time with Lelouch, who was able to forgive and accept her, came to an end, but the memories created with him has, without doubt, saved her from eternal loneliness."
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(And the most compelling evidence of all. Really, TRY to argue with C.C. ...)
C.C.'s Final monologue - Zero Requiem DVD
"There was a boy.
He obtained the power to change the world, to create a new order.
The world was terrified of him, hated him.
... But, I know.
He, who passed away with a smile.
So. This is not a tragedy.
And yet, at the time, the night came where I sobbed with a sadness I wouldn't resist.
I lament. A dirge. The Zero Requiem..."
