Search for pattern in files using system grep. Args: pattern: Pattern to search for paths: File or directory paths to search in (string or list of strings) ignore_case: Case-insensitive matching (-i) before_context: Number of lines before match (-B) after_context: Number of lines after match (-A) context: Number of context lines around match (equal before/after) max_count: Stop after N matches (-m) fixed_strings: Treat pattern as literal text, not regex (-F) recursive: Search directories recursively (-r) regexp: Use regular expressions for pattern matching invert_match: Select non-matching lines (-v) line_number: Show line numbers (-n) file_pattern: Pattern to filter files (e.g., "*.txt") Returns: JSON string with search results