d.mon: Fix SIM115 linter warnings by using context managers for file operations

[arohanajit]

This PR addresses SIM115 linter warnings by replacing manual file handling with context managers in d.mon/render_cmd.py, ensuring proper resource management.

Changes:

• Use with open(...) context managers in update_cmd_file, read_stdin, and temporary file creation.
• Remove redundant fd is None checks (Python’s open() raises exceptions on failure).
• Simplify temporary file creation with appropriate suffixes and delete=False where needed