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-attach.test.js

@@ -55,6 +55,8 @@ describe("pty-attach", () => {
 
   it("accepts a well-formed runId", () => {
     assert.doesNotThrow(() => ptyAttach.validateRunId("agenthub-lane-42"));
+    assert.doesNotThrow(() => ptyAttach.validateRunId("agenthub-lane-42-3"));
+    assert.throws(() => ptyAttach.validateRunId("agenthub-lane-42-4"), /EBADRUNID/);
   });
 
   it("turns tmux mouse mode on for the session it attaches to", () => {