The exec tool previously passed the full parent process environment to
child processes, which meant LLM-generated commands could access secrets
stored in env vars (e.g. API keys from EnvironmentFile=).
Switch from subprocess_shell with inherited env to bash login shell
with a minimal environment (HOME, LANG, TERM only). The login shell
sources the user's profile for PATH setup, making the pathAppend
config option a fallback rather than the primary PATH mechanism.
