fix: resolve AttributeError when appending to existing field

[echovx]

SUMMARY

Fixed AttributeError: 'str' object has no attribute 'append' in src/llm.py.

ISSUE

The original code incorrectly assumed existing fields were always lists, whereas the first extracted value is a string.

SOLUTION

This PR implements a type check using isinstance to handle the dynamic conversion from a single string to a list.

Fixes #325