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__/LaneDiffView.test.tsx

@@ -59,11 +59,28 @@ describe("LaneDiffView — side by side", () => {
     expect(screen.getByTestId("lane-diff-split-right").className).toContain("overflow-auto");
   });
 
-  it("bounds its own height so the horizontal bar stays on screen", () => {
-    // A stacked review list caps each file; the single-file view takes the
-    // height its parent gives it. Either way the diff scrolls itself rather
-    // than growing a 2000-line container whose scrollbar is off the bottom.
+  it("caps a review diff only when it windows; the single-file view fills its parent", () => {
+    // Unwindowed, a review-list diff grows so the PANEL is the one vertical
+    // scroller (nested 60vh boxes made hunk navigation land nowhere); a
+    // windowed one keeps the cap because windowing needs its own scroller.
     const { rerender } = render(<LaneDiffView file={FILE} layout="unified" />);
+    expect(screen.getByTestId("lane-diff-view").className).not.toContain("max-h-[60vh]");
+
+    const big: LaneChangeFile = {
+      ...FILE,
+      hunks: [
+        {
+          ...FILE.hunks![0]!,
+          lines: Array.from({ length: WINDOW_THRESHOLD + 1 }, (_, i) => ({
+            type: "ctx" as const,
+            oldLine: i + 1,
+            newLine: i + 1,
+            text: `line ${i}`,
+          })),
+        },
+      ],
+    };
+    rerender(<LaneDiffView file={big} layout="unified" />);
     expect(screen.getByTestId("lane-diff-view").className).toContain("max-h-[60vh]");
 
     rerender(<LaneDiffView file={FILE} layout="unified" fill />);
@@ -379,3 +396,66 @@ describe("LaneDiffView — windowing", () => {
     expect(container.textContent).toContain("row 4000");
   });
 });
+
+describe("LaneDiffView — hunk review bar navigation", () => {
+  const hunk = (n: number) => ({
+    header: `@@ -${n},1 +${n},1 @@`,
+    context: "",
+    oldStart: n,
+    newStart: n,
+    lines: [
+      { type: "del" as const, oldLine: n, newLine: null, text: `old ${n}` },
+      { type: "add" as const, oldLine: null, newLine: n, text: `new ${n}` },
+    ],
+  });
+  const three: LaneChangeFile = { ...FILE, hunks: [hunk(1), hunk(20), hunk(40)] };
+
+  // jsdom lays nothing out: give the ONE real scroller a scrollable box and
+  // put hunk k's `@@` row k×100px below its top, wherever it has scrolled to.
+  function renderInScroller() {
+    const view = render(
+      <div data-testid="scroller" style={{ overflowY: "auto" }}>
+        <LaneDiffView file={three} layout="unified" hunkActions={() => <span />} />
+      </div>
+    );
+    const scroller = screen.getByTestId("scroller");
+    Object.defineProperty(scroller, "scrollHeight", { value: 1000 });
+    Object.defineProperty(scroller, "clientHeight", { value: 100 });
+    scroller.getBoundingClientRect = () => ({ top: 0 }) as DOMRect;
+    for (const el of view.container.querySelectorAll<HTMLElement>("[data-hunk-index]")) {
+      const k = Number(el.dataset.hunkIndex);
+      el.getBoundingClientRect = () => ({ top: k * 100 - scroller.scrollTop }) as DOMRect;
+    }
+    return scroller;
+  }
+
+  it("steps to the NEXT hunk and back to the PREVIOUS one, not the first", () => {
+    const scroller = renderInScroller();
+    const next = screen.getAllByRole("button", { name: "Next change" });
+    const prev = screen.getAllByRole("button", { name: "Previous change" });
+
+    fireEvent.click(next[0]!); // bar 1 → hunk 2
+    expect(scroller.scrollTop).toBe(100);
+    fireEvent.click(next[1]!); // bar 2 → hunk 3
+    expect(scroller.scrollTop).toBe(200);
+    fireEvent.click(prev[2]!); // bar 3 → hunk 2, not hunk 1
+    expect(scroller.scrollTop).toBe(100);
+  });
+
+  it("counts the hunks and disables the arrows at either end", () => {
+    renderInScroller();
+    const bars = screen.getAllByTestId("lane-hunk-bar");
+    expect(bars.map((b) => b.textContent)).toEqual([
+      expect.stringContaining("1 of 3"),
+      expect.stringContaining("2 of 3"),
+      expect.stringContaining("3 of 3"),
+    ]);
+    expect(screen.getAllByRole("button", { name: "Previous change" })[0]).toBeDisabled();
+    expect(screen.getAllByRole("button", { name: "Next change" })[2]).toBeDisabled();
+  });
+
+  it("leaves an unwindowed review diff uncapped, so the panel is its only scroller", () => {
+    render(<LaneDiffView file={three} layout="unified" />);
+    expect(screen.getByTestId("lane-diff-view").className).not.toContain("max-h-[60vh]");
+  });
+});