feat(workspace): the Changes panel's repo sections are an accordion

With several repos stacked in the Changes panel, only one section is open at
a time: the first starts open, opening another folds the rest so it gets the
whole height, and clicking the open one folds it. A chat's open-file request
opens the section holding the file. Folded sections stay mounted (CSS), so an
unsaved edit survives.

client/src/pages/Workspace.tsx

@@ -1002,8 +1002,11 @@ export function Workspace() {
   const showRoot = !inner || inner.root || repos.length === 0;
   // A file the chat asks for lies in the inner repo whose path prefixes it.
   const repoOf = (p: string) => repos.find((r) => p.startsWith(`${r.path}/`));
-  // Sections folded shut ("" = the workspace folder, else the folder's id).
-  const [foldedSources, setFoldedSources] = useState<Set<string>>(new Set());
+  // The stacked sections are an accordion: ONE is open at a time ("" = the
+  // workspace folder, `repo:<path>` an inner repo, else the folder's id).
+  // `undefined` = the first section (also when the open one is gone); `null`
+  // = every section folded.
+  const [openSource, setOpenSource] = useState<string | null | undefined>(undefined);
   // Right-click on a section: close extra folders — which REMOVES them from the
   // workspace (the folder stays on disk). The workspace folder never closes.
   const sourceCtx = useContextMenu<string>();
@@ -1062,11 +1065,7 @@ export function Workspace() {
       // Its path is the workspace folder's: unfold that section (or the
       // inner repo's that holds it).
       const repo = repoOfRef.current(path);
-      setFoldedSources((prev) => {
-        const next = new Set(prev);
-        next.delete(repo ? `repo:${repo.path}` : "");
-        return next;
-      });
+      setOpenSource(repo ? `repo:${repo.path}` : "");
       setChangesOpen(true);
       setChangesRequest((r) => ({ path, nonce: (r?.nonce ?? 0) + 1 }));
     };
@@ -1792,10 +1791,17 @@ export function Workspace() {
                       // One repo only: the one panel, as it always was.
                       if (sections.length === 1) return panel;
                       // Else every repo is a section, stacked like VS Code's
-                      // Source Control; the open ones share the height. A
+                      // Source Control — an accordion: opening one folds the
+                      // others, so the open one has the whole height. A
                       // folded one stays mounted (CSS), so an unsaved edit in
                       // it survives.
-                      const folded = foldedSources.has(key);
+                      const openKey =
+                        openSource === null
+                          ? null
+                          : sections.some((x) => x.key === openSource)
+                            ? openSource
+                            : sections[0]!.key;
+                      const folded = key !== openKey;
                       return (
                         <section
                           key={key}
@@ -1811,13 +1817,7 @@ export function Workspace() {
                             type="button"
                             aria-expanded={!folded}
                             onContextMenu={(e) => sourceCtx.open(e, key)}
-                            onClick={() =>
-                              setFoldedSources((prev) => {
-                                const next = new Set(prev);
-                                if (!next.delete(key)) next.add(key);
-                                return next;
-                              })
-                            }
+                            onClick={() => setOpenSource(folded ? key : null)}
                             className="flex shrink-0 items-center gap-1 py-1 text-left text-xs font-medium text-fg-secondary hover:text-fg-primary"
                           >
                             {folded ? (

client/src/pages/__tests__/Workspace.changesPanel.test.tsx

@@ -288,11 +288,19 @@ describe("Workspace — the Changes panel's width", () => {
     expect(sections.map((s) => s.querySelector("button")!.textContent)).toEqual(["alpha", "shop"]);
     expect(api.lanes.git).toHaveBeenCalledWith(1);
     expect(api.lanes.git).toHaveBeenCalledWith({ lane: 1, source: 7 });
-    // A folded section keeps its panel mounted, only hidden.
+    // An accordion: the first section starts open, the others folded.
     const toggle = sections[0]!.querySelector("button")!;
-    fireEvent.click(toggle);
+    const shop = sections[1]!.querySelector("button")!;
+    expect(toggle).toHaveAttribute("aria-expanded", "true");
+    expect(shop).toHaveAttribute("aria-expanded", "false");
+    // Opening one folds the other; a folded section keeps its panel mounted.
+    fireEvent.click(shop);
+    expect(shop).toHaveAttribute("aria-expanded", "true");
     expect(toggle).toHaveAttribute("aria-expanded", "false");
     expect(sections[0]!.querySelector('[data-testid="lane-changes"]')).not.toBeNull();
+    // Clicking the open one folds it too.
+    fireEvent.click(shop);
+    expect(shop).toHaveAttribute("aria-expanded", "false");
     // A chat's file is the workspace folder's: its section unfolds.
     act(() => {
       openInChanges(1, "/workspace/alpha", "/workspace/alpha/src/b.ts");
@@ -390,13 +398,13 @@ describe("Workspace — the Changes panel's width", () => {
     fireEvent.click(screen.getByTestId("panel-git-source-rescan"));
     expect(api.lanes.repos).toHaveBeenLastCalledWith(2, true);
     // A chat's file inside an inner repo opens in that repo's section.
-    fireEvent.click(sections[1]!.querySelector("button")!);
     expect(sections[1]!.querySelector("button")).toHaveAttribute("aria-expanded", "false");
     act(() => {
       openInChanges(2, "/workspace/beta", "/workspace/beta/apps/web/src/x.ts");
     });
     await settle();
     expect(sections[1]!.querySelector("button")).toHaveAttribute("aria-expanded", "true");
+    expect(sections[0]!.querySelector("button")).toHaveAttribute("aria-expanded", "false");
     expect(api.lanes.gitFile).toHaveBeenCalledWith(
       { lane: 2, repo: "apps/web" },
       "src/x.ts",