The else branch in inject_background_results is never reached in the
current agent_loop call pattern, where messages[-1] is always a user
message. Kept for independent reuse or future wake-up scenarios.
Every chapter REPL treated an empty line as a quit command and exited with
status 0 without a word, while the banner only promises "Type q to quit".
Under VS Code's debugger the Python extension's environment-activation line
is often injected into stdin after the program started, so the first thing
the REPL read was a blank line and the chapter vanished right after its
banner.
An empty line is now ignored and the prompt comes back; q and exit still
quit, and EOF/Ctrl-C still end the loop. s17 already behaved this way. A
static test keeps every chapter's quit check free of the empty string.
Fixes#567
Wu Shuwen