fix(s04): use early-continue idiom in problem code block

The problem block used a nested `if block.type == "tool_use":` while the
solution block and code.py use `if block.type != "tool_use": continue`.
This made the problem->solution diff show two changes instead of the one
the section teaches (the README states only one place in the loop changed).
Align all three README variants (zh/en/ja) to the early-continue idiom.

s04_hooks/README.en.md

@@ -21,7 +21,8 @@ def agent_loop(messages):
     while True:
         # ... LLM call ...
         for block in response.content:
-            if block.type == "tool_use":
+            if block.type != "tool_use":
+                continue
             log_to_file(block)          # added a line
             check_permission(block)     # added a line
             notify_slack(block)         # added another line

s04_hooks/README.ja.md

@@ -21,7 +21,8 @@ def agent_loop(messages):
     while True:
         # ... LLM call ...
         for block in response.content:
-            if block.type == "tool_use":
+            if block.type != "tool_use":
+                continue
             log_to_file(block)          # 一行追加
             check_permission(block)     # 一行追加
             notify_slack(block)         # さらに一行追加

s04_hooks/README.md

@@ -21,7 +21,8 @@ def agent_loop(messages):
     while True:
         # ... LLM call ...
         for block in response.content:
-            if block.type == "tool_use":
+            if block.type != "tool_use":
+                continue
             log_to_file(block)          # 加一行
             check_permission(block)     # 加一行
             notify_slack(block)         # 又加一行