feat: Add -f flag for single file analysis

[RuneCode14]

Summary

Adds a -f flag for analyzing single malware samples without manual temp directory setup.

Problem

Users often want to analyze a single file but -m expects a directory, forcing manual temp directory creation.

Solution

# Old way (manual temp dir)
mkdir -p /tmp/work && cp sample.exe /tmp/work/
./yargen -m /tmp/work -a "Author"

# New way (simple)  
./yargen -f sample.exe -a "Author"

The -f flag:

• Creates a temp directory internally
• Copies the file
• Runs analysis
• Cleans up automatically

Also prints a recommendation message about using server mode for multiple samples.

Changes

• Added -f string flag for single file path
• Added printSingleFileRecommendation() function
• Updated validation to accept either -m OR -f
• Added proper cleanup with defer os.RemoveAll(tempDir)