```python
# /// intent
# goal   = "Replace every 'code=XNN' with 'code=[XNN]' in logs/app.log and write the result to logs/app.marked.log."
# reads  = ["logs/app.log"]
# writes = ["logs/app.marked.log"]
# output = "none"
# ///

import re
from pathlib import Path

def process_log(source, target):
    """Read log, replace code patterns, write result."""
    text = Path(source).read_text()
    result = re.sub(r'code=([A-Z])(\d+)', lambda m: f"code=[{m.group(1)}{m.group(2)}]", text)
    Path(target).write_text(result)

process_log("logs/app.log", "logs/app.marked.log")
```