Add File Modification Timestamps to Output Headers

Description

Add file modification timestamps to each file header in the output to provide context about when files were last modified. This will help users and LLMs understand development timeline of different files in the repository.

Current Behavior

File headers currently look like:

### File: src/main.py
```python
# file content here

What we want

### File: src/main.py (Modified: 2024-01-15 14:30:22)
```python
# file content here