Fix shell escaping for Slurm env var values

[Pr030304]

Summary

Fix shell escaping for environment variable values in generated Slurm job scripts.

Context

Fixes #17

SlurmJobScript.shell_env_setup interpolates env var values directly into single-quoted shell strings. Values containing a single quote produce invalid shell syntax and can break Slurm job startup.

Changes Made

• shell-escape env var values when rendering export lines

Before fix, it looked like:

export MY_VAR='O'Reilly'

After fix, it looks like:

export MY_VAR='O'"'"'Reilly'

Testing

• Manually reproduced the bug by generating a script with {"MY_VAR": "O'Reilly"}
• Verified the generated script is shell-safe after the fix