fix(maestro): fold the sole root sidebar folder into its own children

~ never distinguishes anything from a sibling — nothing else sits
beside it — so it showed as its own header even when it immediately
branched into multiple real projects. Now it folds into each child
instead, at the top only; a real divergence further down still gets
its own header.

web/src/tree.test.ts

@@ -39,6 +39,16 @@ describe("buildTree", () => {
     expect(names(root)).toEqual(["~/git/naps62/maestro"]);
   });
 
+  it("folds the sole root folder into each of its own children when it branches", () => {
+    const root = buildTree(
+      [session("a", "/home/n/subvisual/bullish"), session("b", "/home/n/tea/maestro")],
+      "/home/n",
+      "",
+    ) as TreeFolder;
+
+    expect(names(root).sort()).toEqual(["~/subvisual/bullish", "~/tea/maestro"]);
+  });
+
   it("stops collapsing where a directory branches", () => {
     const root = buildTree(
       [session("a", "/home/n/git/one"), session("b", "/home/n/git/two")],