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
reactive_compact summarized the full message history before slicing off
the recent tail, so the kept tail was summarized and then re-attached
verbatim, and the emergency prompt-too-long path re-summarized the whole
oversized context. Move the summarize_history call below the tail
computation (including the tool_use/tool_result pair adjustment) and
summarize only messages[:tail_start], so the summary covers older history
while the recent tail stays verbatim.
Applied to the duplicated function in s08, s09, and s20, with the s08
README snippets (en/zh/ja) updated to match. Adds tests covering the
normal path and the case where a tool pair straddles the tail boundary.
Fixes#350
