fix(tiles): "Open group as tiles" keeps the focused session focused

With the grid already open, openGroupAsTiles closed it (which sets
activeSessionId to null, so re-parking does not snapshot the parked
terminal) and only then chose the focus, so the active session was never
"in the group" and the group's first session always took focus. The
picker's Open chooses before closing. Now both do: the focused session
keeps focus when the group holds it, otherwise the group's first session
gets it.

tile-grid-entry-points covers both cases, with the grid open and closed;
the open-grid case failed before this change.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/tile-grid-entry-points.test.ts

@@ -213,6 +213,28 @@ describe('"Open group as tiles"', () => {
     expect(app._tileGrid.ids).toEqual(['s-c', 's-a']);
   });
 
+  it('keeps the focused session focused when the group holds it, open grid or not', () => {
+    for (const gridOpen of [false, true]) {
+      resetGridHarness();
+      const app = makeGridApp(IDS);
+      withGroups(app);
+      if (gridOpen) app.openTileGrid(['s-b', 's-a'], { focusedId: 's-a' });
+      else app.activeSessionId = 's-a';
+      app.openGroupAsTiles('g1');
+      expect(app._tileGrid.ids).toEqual(['s-c', 's-a']);
+      expect(app._tileGrid.focusedId).toBe('s-a');
+      expect(app.activeSessionId).toBe('s-a');
+    }
+  });
+
+  it("focuses the group's first session when it does not hold the focused one", () => {
+    const app = makeGridApp(IDS);
+    withGroups(app);
+    app.openTileGrid(['s-b']);
+    app.openGroupAsTiles('g1');
+    expect(app._tileGrid.focusedId).toBe('s-c');
+  });
+
   it('is not offered where the grid cannot open', () => {
     const app = makeGridApp(IDS);
     const actions = withGroups(app);