Try "I think, regarding code maintainability, open bugs, <other metric>, I've done well, and the project has advance steadily since I arrived, but am eager to h

Try "I think, regarding code maintainability, open bugs, <other metric>, I've done well, and the project has advance steadily since I arrived, but am eager to hear how I may further improve my code." -- basically politely point out your contributions to the CTO, and make the senior developer back-up his claims, cloaked in a call for advice. If you can show you can work well, even with adversarial colleagues, without sowing strife in the company, a reasonable CTO might very well decide to keep you and get rid of the senior dev. But whether your CTO will actually do that, and if you should stay, I cannot say.