Follow-up to the salvaged #52552 and #53083 commits:
- Rework the Matrix PLATFORM_HINTS entry around what the adapter actually
emits: headings, numbered lists, blockquotes, strikethrough-free markdown
all render (the adapter converts them to sanctioned HTML). Keep the
genuinely valuable guidance: no Markdown tables (Element X / Beeper /
mobile clients don't render HTML tables — cells collapse into one line),
no spoilers/checkboxes/~~strikethrough~~ (not converted by
python-markdown), prefer descriptive link text.
- Regression test: hint must steer models away from tables.
- Fix test_long_response_split_preserves_thread_context to derive its
payload size from the adapter's configurable limit instead of assuming
the old hardcoded 4000.
- Document matrix.max_message_length in the Matrix docs page.
- Contributor mapping for RKelln.
