Add a configurable MaxDiffSize field to Client that limits how much
data GetPullRequestDiff will read into memory. The default is 10 MB
(DefaultMaxDiffSize). When the diff exceeds the limit, ErrDiffTooLarge
is returned, allowing callers to skip position translation gracefully.
Implementation uses io.LimitReader to read maxBytes+1, detecting
overflow without buffering the entire response. Setting MaxDiffSize
to -1 disables the limit entirely.
Closes#92
claw