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
extract_text() is new in s06 (listed in the docstring under "Changes
from s05") but was defined under the "FROM s02-s05 (unchanged)" banner.
Move it into the "NEW in s06: Subagent" section, directly above its only
caller spawn_subagent(). Pure move, no logic change.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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