In single-query (-q) mode, the assistant's final answer was printed and
then immediately erased by _print_exit_summary() — which unconditionally
called _clear_terminal_on_exit() (ESC[3J ESC[2J ESC[H]). The answer was
present in the session store but invisible in the terminal.
The clear is only needed for interactive TUI teardown (#38928) where
prompt_toolkit chrome must be cleaned up. Add a clear_screen parameter
to _print_exit_summary() (default True, preserving interactive behavior)
and pass False from the single-query call site so the answer stays
visible above the exit summary.
Regression tests cover:
- clear_screen=True (default) calls _clear_terminal_on_exit()
- clear_screen=False skips the clear
- Single-query -q path passes False end-to-end
- Interactive path still clears (preserving #38928)
