fix(shortcuts): Ctrl+W no longer closes a session

Close Session was bound to Ctrl+W by default. Ctrl+W is delete-word in
every shell, readline prompt and agent CLI, so muscle memory killed the
session (its tmux pane and CLI, with no confirm) mid-sentence, and with
the split pane open it was not even the pane being typed in.

Close Session now has no default key: the capture-phase handler lets
Ctrl+W through and xterm sends ^W to whichever pane is focused. The
action stays in the registry and can be bound in App Settings ->
Shortcuts; the shortcut overlay shows it as not bound. The Help modal,
CLAUDE.md, the split and tile-grid specs and three wiki pages stop
advertising Ctrl+W as kill. Owner decision (tile-grid decision 5).

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

docs/wiki/The-Dashboard.md

@@ -64,7 +64,7 @@ reloading while a permission prompt is blocking does not lose the red tab.
 | Jump to tab N                   | `Alt+1` to `Alt+9` (the number on the tab)              |
 | Next / previous                 | `Ctrl+Tab`, `Alt+[`, `Alt+]`                            |
 | Move the active tab             | `Ctrl+Shift+{`, `Ctrl+Shift+}`                          |
-| Close                           | `Ctrl+W`                                                |
+| Close                           | The tab's close control (no key by default)            |
 | Find any session, open or past  | `Ctrl+K` (also `Cmd+K` and `Alt+K`)                     |
 
 Tabs can also be dragged to reorder.