LLMs routinely emit malformed Markdown (missing heading spaces, glued `---`, unaligned table pipes) that prompt rules cannot reliably prevent. Add a zero-token, regex-only MarkdownNormalizer applied on the FinalAnswerNode convergence path before persistence / channel delivery. It is code-fence aware, idempotent, and conservative (em-dash `---`, `#5`-style refs, stray prose pipes are left untouched). RETURN_DIRECT verbatim output and approval-wait paths return earlier and are unaffected.
Closes#274
