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>

src/web/public/index.html

@@ -807,7 +807,6 @@
           <section class="shortcut-section">
             <h4>Session</h4>
             <div class="shortcuts-grid">
-              <div><kbd>Ctrl</kbd>+<kbd>W</kbd></div><div>Close Session</div>
               <div><kbd>Ctrl/Cmd/Option</kbd>+<kbd>K</kbd></div><div>Find Open Session</div>
               <div><kbd>Ctrl</kbd>+<kbd>Tab</kbd></div><div>Next Session</div>
               <div><kbd>Alt/Option</kbd>+<kbd>[</kbd> / <kbd>Alt/Option</kbd>+<kbd>]</kbd></div><div>Previous / Next Session</div>