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/styles.css

@@ -8520,6 +8520,12 @@ kbd {
   color: var(--text-dim);
 }
 
+/* An action with no key bound (Close Session by default): say so, quietly. */
+.shortcut-overlay-unbound {
+  font-size: 0.75rem;
+  font-style: italic;
+}
+
 .shortcut-overlay-footer {
   margin-top: 0.75rem;
   padding-top: 0.75rem;