Export student-facing feedback to per-repository markdown files
export_comments.RdWrites each student repository's public feedback to
<comments>/<repo>.md under the project root, using the project's
configured comments directory (comments/ when none is configured; it is
created when missing). Each graded question appears as a heading, in
template order, followed by a bulleted markdown list of its selected
rubric item descriptions and its public comment. Private comments are
never exported, and a repository with no public feedback gets no file.