Revert the worker pool, dispatcher, flags, and related scaffolding added
on top of the initial cap guard. Post-#9046, the only remaining CLI
callers of Vcs.diff are one-shot review opens, so the worker offload is
no longer needed. The freeze repro is eliminated by the input caps in
DiffEngine.shouldSkip alone.
Shrinks the fork diff from 28 files (+1063) to 6 files (+199) and
reduces shared opencode files with kilocode_change markers to 2.
Alex Alecu
a7e06d6605