docs(tiles): the tile grid in CLAUDE.md, the invariants, the wiki and the Help modal

- CLAUDE.md: a Tile grid paragraph beside the split-pane one (parking, the
  one load queue, the selection and close rules, chords, dividers, auto-join,
  the exited-agent case), tile-grid.js (7.6) in the load order, the
  desktop-gated header markers and the Tiles picker in the z-index stack.
- docs/architecture-invariants.md#tile-grid: the mechanisms and the reason
  behind each rule; the split section now says where a waiting grid load
  differs and that every capture carries a deadline.
- docs/wiki/Tile-Grid.md: the user manual page (turning it on, the ways in, a
  tile's header, keys, leaving, persistence, Split), linked from the sidebar,
  The Dashboard, Keyboard Shortcuts and Settings Reference.
- The Help modal lists the tile chords (pinned in help-modal-shortcuts.test).
- docs/tile-grid-plan.md: status updated, and an "as built" list of where PR 2
  went another way than the spec.

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

docs/wiki/The-Dashboard.md

@@ -118,6 +118,7 @@ The right side of the header. Almost all of these are off until you enable them
 | Cron ⏰                | Off                | Scheduled jobs.                                                                   |
 | Multi-monitor          | Off, macOS         | Opens a window spanning every display.                                            |
 | Split                  | Off, desktop only  | View a second session beside the active one, with a draggable divider.           |
+| Tiles                  | Off, desktop only  | Up to nine live sessions side by side. See [Tile Grid](Tile-Grid).               |
 | Tunnel indicator       | When a tunnel runs | Cloudflare tunnel status.                                                        |
 | Admin panel            | Multi-user only    | User administration.                                                              |