[Saksham1105]

Summary

• Add safe, idempotent cleanup for \ShellSession\ temporary directory and output files
• Implement context-manager support (_enter_\ / _exit_) on \ShellSession\
• Register process-exit cleanup via \�texit.register(session.cleanup)\ in
  egister_shell_tool\
• Add comprehensive regression tests in \ ests/test_shell_tool.py\

Testing

• Focused test suite: .venv\Scripts\python.exe -m pytest tests/test_shell_tool.py\ (21 passed)
• Full test suite: .venv\Scripts\python.exe -m pytest tests/\ (847 passed, 2 skipped)
• Ruff lint check: .venv\Scripts\ruff check devagent/ tests/\ (All checks passed!)

Closes #99