fix: an empty REPL line no longer quits the chapter silently

[L4XB]

Fixes #567.

Every chapter REPL treated an empty line as a quit command (in ("q", "exit", "") / {"q", "exit", ""}) and exited with status 0 without printing anything, while the banner only promises Type q to quit. As the issue describes, VS Code's Python extension often injects its environment-activation line into stdin after the program started, so under the debugger the first thing the REPL read was a blank line and the chapter vanished right after its banner.

Change (16 chapters, s01–s16; s17 already did this): an empty line is skipped and the prompt comes back; q / exit still quit; EOF / Ctrl-C still end the loop. In s13_agent_teams the payload is None case (CLI quit event) keeps breaking out, and only the empty payload is skipped.

if not query.strip():
    continue
if query.strip().lower() in ("q", "exit"):
    break

Test: tests/test_repl_quit_commands.py parses every s*/code.py and asserts that each quit check exists, that "" is not among its commands, and that the blank-input skip precedes it (s17 excluded, it already had the guard in the other order). pytest tests passes except test_goal_loop.py::test_bash_output_keeps_exit_code_when_the_tail_is_trimmed, which fails identically on main on this machine.