The migration from tiptap-markdown to @tiptap/markdown in 38e92040
broke comment creation. The old package stored getMarkdown() on
editor.storage.markdown, but the official @tiptap/markdown extension
adds it directly to the editor instance (editor.getMarkdown()).
This caused getEditorMarkdown() to always return "", making the
submit button permanently disabled and preventing any comments.
Also fix stale submitting ref in useIssueTimeline dependency array.
9ceea9c17e