Defer CommandShell executable checks until a command runs

[RulaHallak]

Summary

• Defer the CommandShell executable check from initialization to execute(), so a missing shell is reported when a command runs instead of when the object is created.
• Construct SlurmSystem.cmd_shell with default_factory=CommandShell, so each system instance gets its own shell and model creation does not require /bin/bash to exist up front.
• The Sphinx custom.css registration was committed on this branch and then moved to the documentation pull request, so it is not part of this diff.

Test Plan

• Working tree on rula-changes is clean and matches origin/rula-changes.
• Compared this branch with main using git diff origin/main...HEAD. The net change is limited to:
  • src/cloudai/util/command_shell.py
  • src/cloudai/systems/slurm/slurm_system.py
  • 5 insertions and 6 deletions.
• The automated test suite was not re-run for this pull request.

Additional Notes

• Opened as a draft.
• NVIDIA/cloudai main already contains this CommandShell behavior. Fork main does not, which is why the change still shows up here.
• The stylesheet setting from this branch now lives on Title-case documentation headings and clarify experiment output NVIDIA/cloudai#1089.
• The same head branch also has upstream pull request Rula changes NVIDIA/cloudai#1090.