Add HTML formatted_body to Matrix bot messages so that markdown-style
formatting (code blocks, bold, italic, lists) renders properly in Matrix
clients. Uses the pulldown-cmark crate to convert markdown to HTML and
sets the message format to org.matrix.custom.html.
Story: 188_story_render_matrix_bot_messages_with_html_formatting
Instead of waiting for the full LLM response and sending it as a single
message, stream bot responses to Matrix as they are generated. Paragraphs
are delimited by double-newline boundaries, giving users incremental
feedback while the model is still thinking.
Story: 184_story_stream_bot_responses_to_matrix_on_double_newline_boundaries
