fix(cli): keep markdown streaming prop stable to stop settle flash (#13719)

Flipping the <markdown> streaming prop from true to false when an
assistant text segment settles makes MarkdownRenderable call
updateBlocks(true), which skips every block-reuse path and destroys and
recreates all block renderables. Until tree-sitter re-highlights them
the whole message renders blank/unhighlighted, which users see as the
text flashing at the end of each response. Keep streaming={true} for
the transcript markdown (opencode's TUI does the same); entry.streaming
still drives the spinner glyph.

Co-authored-by: Saoud Rizwan <saoudrizwan@users.noreply.github.com>