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/README.md

@@ -726,6 +726,25 @@ flowchart LR
 
 Renders the hook payload for a single event inline below its row. Scalars appear as `key: value` pairs; objects and arrays render in a terminal-styled code block with a copy button.
 
+#### Tooltips (`components/Tooltip.tsx`)
+
+`GlobalTooltip` is mounted once in `App.tsx` and gives **every** control in the
+app the same designed tooltip — there is no wrapper to add at the call site.
+It listens at the document, and on hover (after a 350 ms rest) or on keyboard
+focus (immediately) it takes the element's native `title`, shows it as an
+anchored bubble with an arrow, and puts the attribute back when the bubble
+closes. Consequences worth knowing:
+
+- **Write `title="…"` exactly as before.** That is the whole API. A control
+  keeps its native tooltip, and its accessible name, when JS has not run.
+- **Opt out with `data-tip-off`** where the browser's own tooltip is wanted.
+- The bubble is built from theme tokens (`TOOLTIP_SURFACE`), so it is legible
+  in light mode; it flips above/below, clamps to the viewport, and closes on
+  scroll, resize, pointer-down or `Escape`.
+- `Tip` (`components/Tip.tsx`) stays separate and cursor-following for
+  expanding a truncated **number** inside a line of text, but draws the same
+  surface so the two read as one thing.
+
 ---
 
 ## Utilities