Adds a git-backed session diff panel to the Control UI, complementing the existing PR status chips.
New sessions.diff gateway method (operator.read): resolves the session checkout and returns structured per-file diffs (status, renames, +/- counts, capped unified patch) of branch + uncommitted + untracked work against the default-branch merge base. Hardened against git textconv execution and hardlinked out-of-tree content leaks; handles repos before their first commit via the empty-tree base.
Control UI renders a "Changes" panel in the chat detail sidebar with collapsible per-file diffs, hunk-gap markers, stat chips, and untracked/binary badges, gated on gateway method advertisement. Schemas additive (Swift models regenerated), 20 locales translated.
Closes#104182
1d72c0f423
