docs: add cron example for pew in

[Mirochill]

Summary

• Document pew in as the non-interactive way to run a command inside an environment.
• Add a crontab-style example for running a Python script in a named environment.
• Clarify that pew workon starts a subshell, so it is not the right command for cron-style one-shot execution.

Validation

• git diff --check HEAD~1..HEAD
• Static README search for the new pew in example
• Tests not run locally; documentation-only change.

Closes #236