feat(agents): styled markdown output and compact color-coded HITL menu

Buffer the agent's token stream and render each turn as markdown with
syntax-highlighted code blocks. Collapse HITL prompts to a compact,
color-coded approve/reject/defer menu that shows the command awaiting
approval, arrow-key selectable in a TTY. Correlate multiple pending
HITLs to their tool calls via a FIFO queue so no approval line is blank.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: SSharma-10