feat: warn if explicit include is shadowed by exclude

[patmllr]

Description
Currently, when an explicitly included file is inside an excluded folder, snib silently drops it because exclude wins.
This is technically correct behavior, but confusing for users since their --include appears to be ignored without any feedback.

Steps to reproduce:

snib --verbose scan --smart --exclude "test" --include "test.py"

Where "test.py" is inside the excluded folder "test".

Expected behavior:

• Snib should warn the user whenever an explicitly included file/folder is shadowed by an exclude pattern
  -> WARNING: Explicit include "test.py" is shadowed by exclude "test"
• This would make it clear why the file does not appear in the final include list.

Actual behavior

• File is excluded silently, user might think their include flag is broken.

Problem:
Going through all files in the detect_pattern_conflicts() function is not a valid option because it's too resource intensive.

Environment:
Snib version: 0.4.3
OS: Windows 10
Python version: 3.13.5