All callers only read from the opened file (io.ReadAll + Close). Using
io.ReadWriteCloser in the interface was misleading and violated the
principle of interface segregation. os.Open returns a read-only file
that satisfies io.ReadCloser but not the write contract implied by
io.ReadWriteCloser. Narrow both the interface and the implementation.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
