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/pages/Workspace.tsx

@@ -40,7 +40,7 @@
  *
  * ----------------------------------------------------------------------------- */
 
-import { useCallback, useEffect, useState, useSyncExternalStore } from "react";
+import { useCallback, useEffect, useMemo, useState, useSyncExternalStore } from "react";
 import { useSearchParams } from "react-router-dom";
 import { useTranslation } from "react-i18next";
 import {
@@ -51,7 +51,6 @@ import {
   Pencil,
   Layers,
   PanelLeftClose,
-  ChevronsLeftRight,
   PanelRightClose,
   Maximize2,
 } from "lucide-react";
@@ -89,24 +88,22 @@ const CHANGES_SIZE_KEY = "agenthub.workspace.changesSize";
 /** The three widths the Changes panel cycles through, then closed. Reading a
  *  diff and watching a console want opposite amounts of room, and which one is
  *  "work" changes several times an hour — hence ONE control that steps
- *  narrow → 50/50 → 70/30 → full → closed, rather than a width button beside a
+ *  30/70 → 50/50 → full → closed, rather than a width button beside a
  *  collapse button that both live in the panel's tightest corner. */
-const CHANGES_SIZES = ["compact", "half", "wide", "full"] as const;
+const CHANGES_SIZES = ["compact", "half", "full"] as const;
 type ChangesSize = (typeof CHANGES_SIZES)[number];
 const CHANGES_WIDTH: Record<ChangesSize, string> = {
-  compact: "w-[28rem] xl:w-[34rem]",
+  compact: "w-[30%]",
   half: "w-1/2",
-  wide: "w-[70%]",
   // The console beside it is HIDDEN, not unmounted: a live run's terminal
   // and a half-typed prompt must survive a round trip through full width.
   full: "w-full",
 };
 /** The icon the one resize control shows AT each size: what the next click
- *  gives you — an even split, a wider panel, then closed. */
+ *  gives you — an even split, the full width, then closed. */
 const CHANGES_NEXT_ICON: Record<ChangesSize, typeof Columns2> = {
   compact: Columns2,
-  half: ChevronsLeftRight,
-  wide: Maximize2,
+  half: Maximize2,
   full: PanelRightClose,
 };
 
@@ -163,7 +160,6 @@ function ConsoleArea({
   selectedLaneId,
   splitView,
   setLayout,
-  setPaneLaneId,
   binaryStatus,
   cwdSuggestions,
   activeRuns,
@@ -179,7 +175,6 @@ function ConsoleArea({
   selectedLaneId: number | null;
   splitView: SplitViewState;
   setLayout: (layout: SplitLayout) => void;
-  setPaneLaneId: (index: number, id: number) => void;
   binaryStatus: Record<LaneTool, { found: boolean; path: string | null }> | null;
   cwdSuggestions: CwdSuggestion[];
   activeRuns: RunListResponse | null;
@@ -191,6 +186,30 @@ function ConsoleArea({
   onLaneIdChange: (id: number | null) => void;
   showToggle?: boolean;
 }) {
+  // Every split pane shows the SELECTED lane, each in its own tmux session
+  // (slot = pane index). Tracking what each pane shows lets the others leave
+  // that run out of their Active runs, so no terminal is attached twice.
+  const [paneRunIds, setPaneRunIds] = useState<(string | null)[]>([]);
+  const onPaneHandle = useCallback((i: number, id: string | null) => {
+    setPaneRunIds((prev) => {
+      if (prev[i] === id) return prev;
+      const next = [...prev];
+      next[i] = id;
+      return next;
+    });
+  }, []);
+  const paneHandlers = useMemo(
+    () => [0, 1, 2, 3].map((i) => (id: string | null) => onPaneHandle(i, id)),
+    [onPaneHandle]
+  );
+  const hiddenFor = useMemo(
+    () =>
+      [0, 1, 2, 3].map((i) =>
+        paneRunIds.filter((id, j): id is string => j !== i && j < splitView.layout && !!id)
+      ),
+    [paneRunIds, splitView.layout]
+  );
+
   return (
     <>
       {showToggle && <SplitLayoutToggle splitView={splitView} setLayout={setLayout} />}
@@ -198,6 +217,7 @@ function ConsoleArea({
         <LaneConsolePane
           lanes={lanes}
           laneId={selectedLaneId}
+          onHandleChange={paneHandlers[0]}
           showLaneSelector={false}
           onLaneIdChange={onLaneIdChange}
           onLaneCreated={onLaneCreated}
@@ -215,13 +235,16 @@ function ConsoleArea({
             splitView.layout === 2 ? "grid-cols-2" : "grid-cols-2 grid-rows-2"
           }`}
         >
-          {splitView.paneLaneIds.map((id, i) => (
+          {Array.from({ length: splitView.layout }, (_, i) => (
             <LaneConsolePane
               key={i}
               lanes={lanes}
-              laneId={id}
-              showLaneSelector
-              onLaneIdChange={(newId) => setPaneLaneId(i, newId)}
+              laneId={selectedLaneId}
+              slot={i}
+              hiddenRunIds={hiddenFor[i]}
+              onHandleChange={paneHandlers[i]}
+              showLaneSelector={false}
+              onLaneIdChange={onLaneIdChange}
               onLaneCreated={onLaneCreated}
               binaryStatus={binaryStatus}
               cwdSuggestions={cwdSuggestions}
@@ -335,16 +358,6 @@ export function Workspace() {
     });
   }, []);
 
-  const setPaneLaneId = useCallback((index: number, id: number) => {
-    setSplitView((prev) => {
-      const paneLaneIds = [...prev.paneLaneIds];
-      paneLaneIds[index] = id;
-      const next = { ...prev, paneLaneIds };
-      writeSplitViewState(next);
-      return next;
-    });
-  }, []);
-
   // Pre-flight: probe binary + active runs + cwd suggestions + lanes on mount
   const refreshLanes = useCallback(async () => {
     try {
@@ -960,7 +973,6 @@ export function Workspace() {
                 selectedLaneId={selectedLaneId}
                 splitView={splitView}
                 setLayout={setLayout}
-                setPaneLaneId={setPaneLaneId}
                 binaryStatus={binaryStatus}
                 cwdSuggestions={cwdSuggestions}
                 activeRuns={activeRuns}