fix(ux): forward Esc and Ctrl-X to the sandbox shell while driving

Two interactive shell apps were unusable in the shared sandbox because the
TUI swallowed their core keys:

- Esc released drive mode and was never mapped in key_to_pty, so vim could
  never leave insert mode. Esc now forwards 0x1b to the PTY; F2 (already a
  drive toggle) is the release key.
- Ctrl-X (owner kill switch) was intercepted globally, so nano could never
  quit. It's now gated on !app.driving — while you hold the shell Ctrl-X
  reaches the PTY (0x18, nano's quit); release with F2 to arm the kill
  switch.

Updated the on-screen hints and /help KEYS cluster to match.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

hh/src/ui.rs

@@ -199,7 +199,7 @@ fn help_clusters(theme: &Theme) -> Vec<HelpCluster> {
                     "list local VirtualBox VMs · a VM's snapshots",
                 ),
                 kv("/sbx gui <vm> [yes]", "alias of /sbx launch vbox gui <vm> [yes]"),
-                kv("/drive  ·  F2", "type into the shared shell (Esc releases)"),
+                kv("/drive  ·  F2", "type into the shared shell (F2 releases; Esc reaches vim)"),
             ],
         },
         HelpCluster {
@@ -273,6 +273,10 @@ fn help_clusters(theme: &Theme) -> Vec<HelpCluster> {
                 kv("Enter", "send chat message"),
                 kv("F1  ·  /help", "toggle this help"),
                 kv("Ctrl-C  (while driving)", "interrupt the running command"),
+                kv(
+                    "Ctrl-X  (owner, not driving)",
+                    "kill switch — revoke all drive + interrupt the shell (while driving it reaches the shell, e.g. nano)",
+                ),
                 kv("PgUp / PgDn", "scroll chat  ·  Home/End = oldest/live"),
                 kv(
                     "PgUp / PgDn  (driving)",