feat(workspace): no double resume, server-picked tabs, terminal a11y

- A resume of a session a live pane already holds is refused (409
  ESESSIONLIVE with error.run_id) and the client opens that pane, pasting
  any typed message there; catalog tools refuse resumeSessionId
  (EUNSUPPORTED) instead of silently starting fresh.
- POST /api/lanes/:id/start takes slot: "auto": the server picks the free
  tab under the lane lock (components/lanes/startSession.ts on the client,
  kept out of WorkspaceHome so the sidebar does not pull react-markdown
  into the main bundle).
- Unsaved whole-file edits are guarded on every exit (Back, another file,
  tab switch, close, lane/session change, beforeunload).
- Closing an AI tab asks first; hiding a pane has its own icon. Tabs and
  pane headers show the session name, a state dot, rename by double-click.
- Terminal: Ctrl+M toggles Tab-moves-focus (no keyboard trap), Cmd+F /
  Ctrl+Shift+F find bar, clickable links (noopener), a write-only OSC 52
  clipboard, a Reconnecting overlay, debounced resizes, optional screen
  reader mode. Alt+1-4 / Alt+\ / Alt+W focus, split and hide panes.
- Changes panel: draggable splitter, wrapping git bar, history paging,
  disarming discard/undo/abort, inline diff comments pasted only into a
  visible pane; drag MIME guard on sidebar rows; IME-safe Enter; Show
  more in the sidebar and Recents; idle dots readable with sr-only text.
- Adds @xterm/addon-clipboard, @xterm/addon-search, @xterm/addon-web-links.

client/src/components/run/PaneGrid.tsx

@@ -8,16 +8,28 @@
  * remount it and drop what it drew. With more than one pane each gets a slim
  * header: drag it to move the pane (drop on another pane's edge to split
  * there, on its middle to swap), pick another live session for it, split it
- * right or down, or close the pane (its session keeps running as a tab).
+ * right or down, or HIDE the pane (its session keeps running as a tab — so
+ * its icon is not the tab's stop X). A header names the pane's session and
+ * shows its state dot, as its tab does.
  * Tabs from `ConsoleTabs` drag in the same way (`startPaneDrag`). A
  * right-click on a header offers the swap, a split with a new terminal and
- * the close as one menu. Dividers resize by pointer or by arrow keys.
+ * the close as one menu. Dividers resize by pointer or by arrow keys; a drag
+ * moves only the line and commits the ratio on release, so the layout is
+ * saved and the terminals refit once, not on every pointer move.
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
 import { useCallback, useEffect, useRef, useState, type ReactNode } from "react";
 import { useTranslation } from "react-i18next";
-import { Check, ChevronDown, Columns2, GripVertical, Rows2, SquareTerminal, X } from "lucide-react";
+import {
+  Check,
+  ChevronDown,
+  Columns2,
+  GripVertical,
+  Minimize2,
+  Rows2,
+  SquareTerminal,
+} from "lucide-react";
 import type { RunHandle } from "../../lib/api";
 import { ContextMenu, MenuItem, MenuSeparator, useContextMenu } from "../ContextMenu";
 import {
@@ -37,6 +49,7 @@ import {
   swapInLayout,
 } from "../../lib/paneLayout";
 import { AiToolIcon } from "../AiToolIcon";
+import { RunDot, type RunState } from "./ConsoleTabs";
 
 export const PANE_DRAG_TYPE = "application/x-agenthub-pane";
 /** The slot being dragged: `dataTransfer` data cannot be read during dragover. */
@@ -283,8 +296,11 @@ export function PaneGrid({
   tools,
   toolLabel,
   onNewSplit,
+  stateOf,
 }: {
   layout: PaneLayout;
+  /** A live AI run's state for its header dot (none for a shell). */
+  stateOf?: (r: RunHandle) => RunState | null;
   /** Every slot to keep mounted; those not in the layout are hidden. */
   mounted: number[];
   /** The workspace's live runs (its tabs). */
@@ -433,6 +449,7 @@ export function PaneGrid({
                 <GripVertical className="h-3.5 w-3.5 shrink-0 opacity-60" aria-hidden />
                 {run && <RunIcon run={run} label={label(run)} />}
                 <span className="min-w-0 truncate">{run ? label(run) : t("panes.empty")}</span>
+                {run && stateOf?.(run) && <RunDot state={stateOf(run)!} />}
                 <PaneMenu
                   testId={`pane-pick-${slot}`}
                   label={t("panes.pick")}
@@ -482,14 +499,14 @@ export function PaneGrid({
                     onClick={() => onChange((l) => closeInLayout(l, slot))}
                     className="flex h-6 w-6 items-center justify-center rounded text-fg-muted hover:bg-surface-4 hover:text-fg-primary"
                   >
-                    <X className="h-3.5 w-3.5" />
+                    <Minimize2 className="h-3.5 w-3.5" />
                   </button>
                 </div>
               </div>
             )}
-            <div className="flex min-h-0 flex-1 flex-col pb-1 pl-3 pr-1 pt-1.5">
-              {renderPane(slot)}
-            </div>
+            {/* No padding here: the terminal paints its own gutter in its
+                palette, so a dark pane never sits in a light-page frame. */}
+            <div className="flex min-h-0 flex-1 flex-col">{renderPane(slot)}</div>
           </div>
         );
       })}
@@ -550,23 +567,31 @@ export function PaneGrid({
   );
 }
 
-/** A split's divider: a 1px line with a wider grab area. */
-function Divider({
+/** A split's divider: a 1px line with a wider grab area. Also the Changes
+ *  panel's splitter (Workspace.tsx), over the whole row (`box` = 0,0,1,1). */
+export function Divider({
   gridRef,
   dir,
   at,
   box,
   onRatio,
+  label,
 }: {
   gridRef: React.RefObject<HTMLDivElement>;
   dir: SplitDir;
   at: number;
   box: Rect;
   onRatio: (ratio: number) => void;
+  /** Accessible name; the panes' own by default. */
+  label?: string;
 }) {
   const { t } = useTranslation("lanes");
-  const [dragging, setDragging] = useState(false);
+  // The ratio under the pointer while dragging: only this line moves until
+  // release, when the layout takes it once.
+  const [live, setLive] = useState<number | null>(null);
+  const dragging = live !== null;
   const row = dir === "row";
+  const pos = live ?? at;
   const ratioAt = (e: React.PointerEvent) => {
     const g = gridRef.current?.getBoundingClientRect();
     if (!g || !g.width || !g.height) return null;
@@ -575,17 +600,22 @@ function Divider({
       : ((e.clientY - g.top) / g.height - box.y) / box.h;
   };
   const style = row
-    ? { left: `${(box.x + box.w * at) * 100}%`, top: `${box.y * 100}%`, height: `${box.h * 100}%` }
-    : { top: `${(box.y + box.h * at) * 100}%`, left: `${box.x * 100}%`, width: `${box.w * 100}%` };
+    ? { left: `${(box.x + box.w * pos) * 100}%`, top: `${box.y * 100}%`, height: `${box.h * 100}%` }
+    : { top: `${(box.y + box.h * pos) * 100}%`, left: `${box.x * 100}%`, width: `${box.w * 100}%` };
+  const clamp = (r: number) => Math.min(0.9, Math.max(0.1, r));
+  const finish = () => {
+    if (live !== null && live !== at) onRatio(live);
+    setLive(null);
+  };
   return (
     <div
       role="separator"
       tabIndex={0}
       aria-orientation={row ? "vertical" : "horizontal"}
-      aria-valuenow={Math.round(at * 100)}
+      aria-valuenow={Math.round(pos * 100)}
       aria-valuemin={10}
       aria-valuemax={90}
-      aria-label={t("panes.resize")}
+      aria-label={label ?? t("panes.resize")}
       className={`group absolute z-10 flex justify-center outline-none ${
         row
           ? "w-2 -translate-x-1/2 cursor-col-resize"
@@ -595,15 +625,15 @@ function Divider({
       onPointerDown={(e) => {
         e.preventDefault();
         e.currentTarget.setPointerCapture?.(e.pointerId);
-        setDragging(true);
+        setLive(at);
       }}
       onPointerMove={(e) => {
         if (!dragging) return;
         const r = ratioAt(e);
-        if (r !== null) onRatio(r);
+        if (r !== null) setLive(clamp(r));
       }}
-      onPointerUp={() => setDragging(false)}
-      onPointerCancel={() => setDragging(false)}
+      onPointerUp={finish}
+      onPointerCancel={() => setLive(null)}
       onKeyDown={(e) => {
         const back = row ? "ArrowLeft" : "ArrowUp";
         const fwd = row ? "ArrowRight" : "ArrowDown";