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