feat(lanes,ui): per-lane git surface — Changes panel, VS Code-style diff, branch-aware Add lane, redesigned tooltips (#34, #35)

- Changes card beside lane detail with five modes: Files tree, overview,
  Review (whole diff, per-file collapse), single file, commit history.
- LaneDiffView: unified / side-by-side with independent scroll on both axes
  and mirrored scrollLeft, expand/collapse context, intra-line emphasis.
- Staged-only commit box; git actions (fetch/pull/push/stage/unstage/
  discard/commit/switch/branch-create/branch-delete) guarded in git-ops.js.
- Server: git-diff.js + git-ops.js behind GET /api/lanes/:id/git/* and
  POST /api/lanes/:id/git/:action; gitFacts gains upstream/ahead/behind.
- Add lane: new/existing branch, current branch, worktree list, prefill
  from the git bar's "New lane here"; /api/run/cwds gains a repo kind.
- GlobalTooltip upgrades every native title into a themed anchored bubble;
  chart/info popovers use theme tokens.

client/src/components/workflows/SessionComplexityScatter.tsx

@@ -115,7 +115,7 @@ function Tooltip({ state }: { state: TooltipState }) {
 
   return (
     <div
-      className="fixed z-50 px-3 py-2 text-xs bg-[#12121f] border border-[#2a2a4a] rounded-lg shadow-xl text-fg-secondary pointer-events-none whitespace-nowrap"
+      className="fixed z-50 px-3 py-2 text-xs bg-surface-2 border border-border rounded-lg shadow-xl text-fg-secondary pointer-events-none whitespace-nowrap"
       style={{
         left: nearRight ? state.x - 12 : state.x + 12,
         top: state.y - 10,
@@ -144,7 +144,7 @@ function Tooltip({ state }: { state: TooltipState }) {
           </span>
         )}
       </div>
-      <div className="mt-1 pt-1 border-t border-[#2a2a4a]">
+      <div className="mt-1 pt-1 border-t border-border">
         <span className="font-medium" style={{ color: statusColor(state.item.status) }}>
           {t(`common:status.${state.item.status}`, { defaultValue: state.item.status })}
         </span>