feat(tiles): the Tiles button opens the grid at once; the picker is on right-click

Owner decision 8 ("when I hit the tiles button, open the tiles already!").
A click with the grid closed now opens it straight away, and Ctrl+Shift+G
runs the same function (toggleTileGrid), so the two cannot drift. What
opens comes from one pure helper, tileGridOpenSet (constants.js):
  a. the grid this tab last had, if any of its sessions survive, opened
     exactly (an open split closes and its sessions do not join);
  b. else an open split's two sessions, Pane A focused;
  c. else the open sessions in tab order (the picker's list: no detached
     ones), up to what the grid takes here (the cap of 6, fewer when the
     window fits fewer), the active session always among them and focused.
A click with the grid open still closes it.

The picker moved to right-click (oncontextmenu, browser menu suppressed).
With the grid open it is preselected with the current tiles, and Open
replaces them. Ctrl/Cmd+click on a tab with the grid closed opens the
toggle's set plus that session. The button's title, the Help modal and
the wiki say right-click chooses which sessions.

Docs: decision 8 and an as-built entry in the spec (Entry points too),
CLAUDE.md, the invariants (#tile-grid, Opening), the wiki's Tile Grid and
Keyboard Shortcuts pages.

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

test/tile-grid-split-coexistence.test.ts

@@ -65,6 +65,22 @@ describe('opening the grid over an open split', () => {
     expect(app.sendResize).toHaveBeenCalledTimes(1); // the split's own opening resize only
   });
 
+  it('a remembered grid wins over an open split: exactly its tiles, the split closed and not merged', () => {
+    const app = makeGridApp(IDS);
+    app.selectSession = vi.fn();
+    app.openTileGrid(['s-c']);
+    app.closeTileGrid({ reselect: false });
+    app.activeSessionId = 's-a';
+    const paneB = openSplit(app);
+
+    app.toggleTileGrid();
+
+    expect(paneB.destroy).toHaveBeenCalledTimes(1);
+    expect(app._splitPane).toBeNull();
+    expect(app._tileGrid.ids).toEqual(['s-c']);
+    expect(app.activeSessionId).toBe('s-c');
+  });
+
   it('an explicit open over a split keeps both split sessions first', () => {
     const app = makeGridApp(IDS);
     openSplit(app);