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/i18n/locales/en/lanes.json

@@ -108,10 +108,10 @@
   "changes.refresh": "Refresh",
   "changes.resize": "Resize the changes panel (next: {{size}})",
   "changes.size.compact": "Narrow",
-  "changes.size.half": "50 / 50",
-  "changes.size.wide": "70 / 30",
-  "changes.size.full": "Full width",
+  "changes.size.half": "Medium",
+  "changes.size.full": "Full screen",
   "changes.stageAll": "Stage all ({{count}})",
+  "changes.unstageAll": "Unstage all ({{count}})",
   "changes.stage": "Stage this file",
   "changes.title": "Changes",
   "changes.truncated": "Some changes were left out to keep this panel responsive.",
@@ -237,5 +237,18 @@
   "worktrees.heading_other": "{{count}} worktrees",
   "changes.stageHunk": "Stage this hunk",
   "changes.undoHunk": "Undo this hunk",
-  "changes.undoHunkConfirm": "Click again to undo this hunk — this cannot be undone"
+  "changes.undoHunkConfirm": "Click again to undo this hunk — this cannot be undone",
+  "changes.stageHunkShort": "Keep",
+  "changes.undoHunkShort": "Undo",
+  "changes.undoHunkShortConfirm": "Confirm undo",
+  "changes.edit": "Edit",
+  "changes.save": "Save",
+  "changes.cancelEdit": "Cancel",
+  "changes.editorLabel": "Edit {{path}}",
+  "changes.prevHunk": "Previous change",
+  "changes.nextHunk": "Next change",
+  "changes.hunkOf": "{{n}} of {{total}}",
+  "changes.noEditBinary": "Binary file — not editable here",
+  "changes.noEditTooBig": "Too large to edit here (over 1 MB)",
+  "changes.noEdit": "Not editable here"
 }