What is SHA-256 in Conjunctive Normal Form? What is the conjunctive normal form (CNF) representation of $\text{SHA-256}(m)=h$, where $m$ is of fixed (or bounded) size and $h$ is 256 bit?
There is none. Conjunctive normal form is for Boolean formulas, which have a single truth value. SHA-256 is not a Boolean formula (SHA-256(x) is not "true" or "false.") Furthermore, CNF is only applicable if you have a fixed number of variables; this is not true for SHA-256, which has variable-length input. At best you could have 256 different formulas, each for one bit of the output, and have $2^{64}+63$ different variables in each formula.

You've probably confused decision problems with function problems. Any algorithm that returns "true" or "false" can be represented, ultimately, as a gigantic Boolean formula. Anything not just returning "true" or "false" cannot, as a Boolean formula returns only "true" or "false." While there is a $2^{64}+63+256$-variable formula that evaluates to "is $x_1\ldots x_{256}$ the SHA-256 hash of the $x_{257}\ldots x_{320}$-bit number stored in $x_{321}\ldots x_{\text{last}}$," the formula is really just one of those things that exists in theory and not in practice.