refactor: summarize only trimmed history in reactive_compact

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

s08_context_compact/README.ja.md

@@ -139,10 +139,10 @@ API がまだ `prompt_too_long`（413）を返すことがある。コンテキ
 ```python
 def reactive_compact(messages):
     transcript = write_transcript(messages)
-    summary = summarize_history(messages)
     tail_start = max(0, len(messages) - 5)
     if _is_tool_result_message(messages[tail_start]) and _message_has_tool_use(messages[tail_start - 1]):
         tail_start -= 1
+    summary = summarize_history(messages[:tail_start])
     return [{"role": "user",
              "content": f"[Reactive compact]\n\n{summary}"}, *messages[tail_start:]]
 ```