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>

docs/wiki/Keyboard-Shortcuts.md

@@ -47,6 +47,7 @@ Anything you copy is cleaned on the way to the clipboard: each line loses the pa
 | `Alt+Shift+Arrows`          | Focus the tile to the left, right, above or below.            |
 | `Alt+Shift+Enter`           | Zoom the focused tile, or restore the grid.                   |
 | `Ctrl`+click / `Cmd`+click a tab | Add that session to the grid.                            |
+| Right-click the Tiles button | Choose which sessions to show as tiles.                  |
 
 While the grid is open, `Ctrl+Tab` and `Alt+[` / `Alt+]` cycle through the tiles, and the
 terminal shortcuts above act on the focused tile. **Remove Focused Tile** has no key by

docs/wiki/Tile-Grid.md

@@ -15,12 +15,14 @@ button in the header, beside Split, and enables `Ctrl+Shift+G`.
 
 ## Opening a grid
 
-- **Tiles button**: with the grid closed it opens a picker, a checkbox per open session in
-  tab order. It starts with the grid you last had (or the session you are on), says how many
-  tiles this window fits, and greys out the rest. **Open tiles** shows them. With the grid
-  open, the same button closes it.
-- **`Ctrl+Shift+G`**: toggles the grid. Opening brings back the grid you last had, else an
-  open split as two tiles, else the session you are on.
+- **Tiles button**: one click shows the tiles straight away. You get the grid you last
+  had; if there is none, an open split as two tiles; otherwise your open sessions in tab
+  order, up to six (fewer if the window is too small), with the session you are on
+  focused. With the grid open, the same button closes it.
+- **Right-click the Tiles button** to choose which sessions: a checkbox per open session in
+  tab order, starting with the tiles you have (or had), greying out the rest once the grid
+  is full. **Open tiles** shows them, replacing what the grid showed.
+- **`Ctrl+Shift+G`**: exactly what a click on the Tiles button does.
 - **`Ctrl`+click (or `Cmd`+click) a tab**: adds that session to the grid and focuses it,
   opening the grid if it was closed. On macOS use `Cmd`: `Ctrl`+click there opens the tab's
   rename instead.