>>279932224
She very clearly and deliberately moved closed to Takina which is why she puts her arm around her and moves even closer afterwards rather than acting like it was a mistake headbutt (which isn't described by the entire text as you can read). It was a conscious choice to put her lips on Takina's neck.

>Chisato giggled, softly putting her arm around Takina from behind to snuggle up to her. She’d slipped under Takina’s quilt.

And as you can see from Chisato sleep-talking, they're having the same dream.