Git diff review pane now tracks explicit fullscreen state and a cwd override

The git diff review pane (the view that shows pending changes for review) now tracks more state internally:

• a cwd field, so process runs use an explicit working directory when one is set
• an isEmpty flag, letting it skip re-checking git status when a session is already known to have no repository
• an epoch counter, which cancels in-progress work to resolve a diff's base when a session is cleared or resumed

Separately, whether the pane is shown fullscreen is now derived from the actual render viewport instead of being assumed, and the pane now only auto-opens when it is explicitly confirmed to be fullscreen.