feat(lanes): split panes on one lane, in-place file editing, hunk review bar

- Split view: every pane shows the selected lane, each in its own tmux
  session (`slot` 0-3 on POST /api/lanes/:id/start; slot 0 stays the
  lane's `run_id`, slots 1-3 are `agenthub-lane-<id>-<slot>`). A pane's
  Active runs hides what the other panes show; stop/reset/remove kill
  every slot. No per-pane lane picker any more.
- Whole-file editor: PUT /api/lanes/:id/git/file (lane-file-write.js)
  with the read path's guards, existing text files only, and a baseSha
  stale check (409 ESTALE). A file it cannot take says why.
- Per-hunk review bar under each hunk's last changed line: prev/next,
  "n of N", Undo, Keep. Review diffs no longer nest their own scroll box
  (unless windowed), so navigation moves the one real scroller.
- Files tree lists untracked-not-ignored files and colours entries by git
  status (M/U/A/D/R), folders holding changes get a dot.
- Changes panel: Unstage all on the single commit/Stage-all row; sizes
  Narrow (30%) / Medium / Full screen, no 70/30.
- Lane card: the needs-you banner is gone (the strip tile keeps it).

client/src/components/lanes/__tests__/LaneCard.test.tsx

@@ -167,9 +167,9 @@ describe("LaneCard rebuilt layout", () => {
     expect(screen.queryByText("48%")).toBeNull();
   });
 
-  it("surfaces a needs-you message", () => {
+  it("leaves the needs-you message to the strip card", () => {
     render(<LaneCard lane={full({ needs_action: "waiting on approval" })} onAction={vi.fn()} />);
-    expect(screen.getByText(/waiting on approval/)).toBeInTheDocument();
+    expect(screen.queryByText(/waiting on approval/)).toBeNull();
   });
 
   it("keeps reset behind the ⋯ menu rather than showing it directly", async () => {