[SECURITY] Strengthen GitHub URL validation and sanitization

[jonasyr]

Description

The current GitHub URL validation only checks for URL prefix (https://github.com/) but doesn't validate the actual repository format or prevent potentially malicious URLs.

Current Behavior

def is_github_repo(self):
    return (self.input_path.startswith("https://github.com/") or
            self.input_path.startswith("git@github.com:"))

Security Concerns

• No validation of repository name format
• Accepts any URL starting with https://github.com/
• No protection against URL injection attacks
• Missing support for authentication options

Expected Behavior

• Strict validation of GitHub repository URL format
• Sanitization of repository names
• Clear error messages for invalid URLs
• Optional support for authentication tokens

Implementation Requirements

def _validate_github_url(self, url):
    """Validate and sanitize GitHub repository URL"""
    import re
    
    # Support both HTTPS and SSH formats
    https_pattern = r'^https://github\.com/([a-zA-Z0-9._-]+)/([a-zA-Z0-9._-]+)(?:\.git)?/?$'
    ssh_pattern = r'^git@github\.com:([a-zA-Z0-9._-]+)/([a-zA-Z0-9._-]+)(?:\.git)?$'
    
    if re.match(https_pattern, url) or re.match(ssh_pattern, url):
        return True
    return False

Files Affected

• codebase_to_text/codebase_to_text.py (lines 586-588, 567-570)

Acceptance Criteria

• Implement strict URL validation with regex patterns
• Add support for .git suffix in URLs
• Validate repository and username formats
• Add comprehensive tests for valid/invalid URLs
• Update documentation with supported URL formats
• Add clear error messages for validation failures

Test Cases to Add

• Valid: https://github.com/user/repo, https://github.com/user/repo.git
• Invalid: https://github.com/, https://github.com/user/, https://github.com/../malicious