Fix [devbox] prompt marker disappearing after 'exec bash'

The previous guard used an exported DEVBOX_PS1_SET env var to avoid
double-prefixing on re-source. But env vars survive 'exec bash'
while PS1 does not — a new bash rebuilds PS1 from .bashrc. Result:
the guard saw DEVBOX_PS1_SET=1, skipped the prefix, and the new
shell ran with bare PS1 (no [devbox] marker).

Replace the env-var guard with a substring check on PS1 itself.
If PS1 already contains '[devbox]' we skip, otherwise we prepend.
Correct in all three cases: first shell (PS1 has no marker → add),
exec bash (fresh PS1 has no marker → add), re-source within same
shell (PS1 still has marker → skip, no doubling).

rootfs/home/developer/.bash_aliases

@@ -76,7 +76,10 @@ fi
 
 # ── Prompt: show [opencode-devbox] tag so it's obvious you're in the container
 # Preserves the default Debian PS1 logic but prefixes with a container marker.
-if [ -n "${PS1:-}" ] && [ -z "${DEVBOX_PS1_SET:-}" ]; then
+# We check for the literal '[devbox]' substring in PS1 rather than relying on
+# an exported guard variable — otherwise `exec bash` inherits the guard but
+# gets a fresh (prefix-less) PS1 from .bashrc, and the prefix would never be
+# re-added in the new shell.
+if [ -n "${PS1:-}" ] && [[ "$PS1" != *"[devbox]"* ]]; then
   PS1='\[\e[38;5;39m\][devbox]\[\e[0m\] '"${PS1}"
-  export DEVBOX_PS1_SET=1
 fi