/bin/bash isn't POSIX

[Dietr1ch]

The codebase targets /bin/bash, but instead should target either /bin/sh if a POSIX shell suffices, or /usr/bin/env bash if bash is necessary.

I see Error: PTY shell exited during startup happens when the @deepseek-ai/dsh-experimental-auto-review plugins tries to start /bin/bash on systems where system-level binaries are limited to POSIX ones.