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).

server/__tests__/pty-run.test.js

@@ -58,6 +58,26 @@ describe("pty-run", () => {
     assert.ok(newSessionCall.includes("opus"));
   });
 
+  it("spawnRun names a split pane's session agenthub-lane-<id>-<slot>, and refuses a slot past 3", () => {
+    tmux.__setExecImpl((args) => {
+      if (args[0] === "has-session") {
+        const e = new Error("no such session");
+        e.status = 1;
+        throw e;
+      }
+      return "";
+    });
+    const handle = pty.spawnRun({ laneId: 42, slot: 2, cwd: "/tmp/repo" });
+    assert.equal(handle.id, "agenthub-lane-42-2");
+    assert.equal(handle.laneId, 42);
+    assert.equal(handle.slot, 2);
+    assert.equal(pty.laneIdFromRunId("agenthub-lane-42-2"), 42);
+    assert.throws(
+      () => pty.spawnRun({ laneId: 42, slot: 4, cwd: "/tmp/repo" }),
+      (err) => err.code === "EBADSLOT"
+    );
+  });
+
   it("spawnRun refuses with ETMUXMISSING, naming the fix, when tmux is not installed", () => {
     // Before this guard the ENOENT escaped from three calls down as
     // `spawnSync tmux ENOENT` and the route answered 400 — blaming the request