Extends SECTION_DEFAULTS so the out-of-the-box TUI shows the turn as
a live transcript (reasoning + tool calls streaming inline) instead of
a wall of `▸` chevrons the user has to click every turn.
Final default matrix:
- thinking: expanded
- tools: expanded
- activity: hidden (unchanged from the previous commit)
- subagents: falls through to details_mode (collapsed by default)
Everything explicit in `display.sections` still wins, so anyone who
already pinned an override keeps their layout. One-line revert is
`display.sections.<name>: collapsed`.
67bfd4b828