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).
joakimp
9bb93025f0