Pattern for extracting @file mentions from input text.
Matches @ followed by one or more path characters or escaped character
pairs (backslash + any character, e.g., \ for spaces in paths).
Uses + (not *) because a bare @ without a path is not a valid
file reference.
FILE_MENTION_PATTERN = re.compile('@(?P<path>(?:\\\\.|[' PATH_CHAR_CLASS '])+)')