feat(tiles): no + in the tile header (owner decision 9)

Owner: "remove the + button from these views". The tile header is now
● name ... ⋯ ⤢ ×. Gone with it, because nothing else used them: the +
menu (openTileAddMenu, closeTileAddMenu and their hooks in closeTileGrid
and the global Escape handler), its "New session in this case" entry and
runInCaseForTiles, the .tile-add-empty rules, the four i18n entries only
the menu showed, and buildTilePickerSessions' exclude argument (only the
menu passed it).

Every other way of adding tiles stays and needed nothing from the menu:
the Tiles button and its right-click picker, Ctrl/Cmd+click on a tab,
dragging a tab onto a tile or an empty slot, "Open group as tiles", and
Run joining the open grid (_joinTileGridFromRun). The picker list, the
cap helper _tileGridLimit and the user-text skip on names are shared and
kept.

Tests: the + menu cases (picker, cap, auto-join, the zh-CN harvest) are
removed; tile-grid-chrome pins the header as exactly ⋯ ⤢ × with no add
menu or runInCaseForTiles left on the app.

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

test/tile-grid-i18n.test.ts

@@ -5,7 +5,7 @@
  *
  * - Runtime strings are HARVESTED from the real tile code (the shared vm
  *   harness, test/mocks/tile-grid-vm.ts) driven through every state that
- *   writes text: the picker (cap and window wording), a tile's +, the Attach
+ *   writes text: the picker (cap and window wording), the Attach
  *   overlay (not attached, attaching, exited, ended), zoom, the header
  *   tooltip, the dividers, the empty slot, every toast, the crash-restart
  *   confirm, the Tiles and Split button titles. Each must translate to text
@@ -14,7 +14,7 @@
  * - Static strings: the shortcut registry's tile entries (overlay and App
  *   Settings list), and index.html run through the real translator in JSDOM
  *   (the Tiles button, the App Settings chips, the Help modal's Tiles rows).
- * - User text stays as typed: session names (tile header, picker, + menu)
+ * - User text stays as typed: session names (tile header, picker)
  *   and group names carry data-i18n-skip, and a session name inside the
  *   confirm passes through the pattern untranslated.
  *
@@ -130,8 +130,6 @@ async function exercise() {
   let pill = 'idle';
   app._sidebarRichRow = () => ({ state: pill, pill, since: { at: 1 } });
   app._mobileOverviewStampText = () => '3m';
-  app.cases = [{ name: 'proj', path: '/w' }];
-  app._mobileOverviewCaseFor = (_dir: string, cases: Array<{ name: string }>) => cases[0] ?? null;
   app.sessions.get('s-2').pid = null;
   app.sessions.get('s-3').paneExit = { status: 3 };
   app.sessions.get('s-4').paneExit = { signal: 9 };
@@ -183,15 +181,6 @@ async function exercise() {
   harvestAll(app, 'zoomed');
   app.zoomTile('s-1');
 
-  // A tile's + on a full grid, from a session in a case and from one outside.
-  app.openTileAddMenu({ stopPropagation: vi.fn(), preventDefault: vi.fn(), currentTarget: null }, 's-1');
-  harvestAll(app, '+ menu, full');
-  app.closeTileAddMenu();
-  app._mobileOverviewCaseFor = () => null;
-  app.openTileAddMenu({ stopPropagation: vi.fn(), preventDefault: vi.fn(), currentTarget: null }, 's-1');
-  harvestAll(app, '+ menu, no case');
-  app.closeTileAddMenu();
-
   // The toasts of a full grid, by the cap and by the window.
   app._joinTileGridFromRun('s-7');
   app.addSessionToTiles('s-7');
@@ -264,7 +253,6 @@ describe('every tile grid string the code puts on screen translates to zh-CN', (
       'Zoom this tile',
       'Restore the grid',
       'Session actions',
-      'Add a session to the grid',
       'Remove tile (the session keeps running)',
       'idle 3m',
       'needs you 3m',
@@ -276,8 +264,6 @@ describe('every tile grid string the code puts on screen translates to zh-CN', (
       'The agent exited (signal 9)',
       'The session ended',
       'It cannot be restarted in place: close it from ⋯ (Close session).',
-      'New session in this case',
-      'This session is not in a case',
       'The grid holds at most 6 tiles: the new session opens on its own',
       'The grid already holds what this window fits (4): the new session opens on its own',
       'The grid already holds what this window fits (4)',
@@ -375,12 +361,12 @@ describe('the static markup through the real translator (JSDOM, zh-CN)', () => {
 });
 
 describe('user text in the tile code', () => {
-  it('session names in a tile header, the picker and the + menu are marked data-i18n-skip', () => {
+  it('session names in a tile header and the picker are marked data-i18n-skip', () => {
     // The harvest saw them only inside skipped subtrees.
     expect(userText.has('Open tiles')).toBe(true);
     expect([...seen.keys()]).not.toContain('s-1');
     const src = read('tile-grid.js');
-    expect(src.match(/setAttribute\('data-i18n-skip', ''\)/g)?.length).toBeGreaterThanOrEqual(3);
+    expect(src.match(/setAttribute\('data-i18n-skip', ''\)/g)?.length).toBeGreaterThanOrEqual(2);
   });
 
   it('the group names of the grouped rail are marked data-i18n-skip', () => {