fix(shell): don't export DEVBOX_HIST_SET so nested shells flush history

The history-flush guard was exported, so it leaked into child processes.
Any nested shell -- crucially each tmux pane (which inherits the tmux
server's env) -- then saw the guard already set and skipped installing
'history -a' in PROMPT_COMMAND. Those shells only persisted history on a
clean exit, so abrupt termination (docker stop, tmux kill-server, SIGKILL)
silently lost their in-memory history. zoxide was less affected (its hook
is installed unguarded and writes immediately).

Make the guard shell-local (drop 'export') so every new interactive shell
re-installs its own per-prompt flush. Add a recreate-sanity-check assertion
that a nested login shell still wires up 'history -a'.

Storage was never the issue: ~/.cache/bash (devbox-shell-history) and
~/.local/share/zoxide (devbox-zoxide) are both persistent named volumes.

rootfs/home/developer/.bash_aliases

@@ -89,9 +89,16 @@ fi
 # we append with a newline separator to avoid the ';;' parse error
 # described at the top of this file. Guarded so repeated sourcing
 # (e.g. `exec bash`) doesn't stack duplicates.
+#
+# The guard MUST stay shell-local (NOT exported): if it leaks into child
+# processes, every nested shell -- crucially each tmux pane, which inherits
+# the tmux server's env -- skips installing `history -a` and only persists
+# history on a clean exit. Abrupt termination (docker stop, tmux kill-server,
+# SIGKILL) then loses that shell's in-memory history. Keeping it unexported
+# means each new interactive shell re-installs its own per-prompt flush.
 if [ -z "${DEVBOX_HIST_SET:-}" ]; then
   PROMPT_COMMAND="${PROMPT_COMMAND:+$PROMPT_COMMAND$'\n'}history -a"
-  export DEVBOX_HIST_SET=1
+  DEVBOX_HIST_SET=1
 fi
 
 # ── Prompt: show [opencode-devbox] tag so it's obvious you're in the container