fix(md-exec): heredoc body containing a ``` line is mistaken for the end of the code block

scripts/md-exec.md

@@ -219,6 +219,17 @@ blah
 EOF
 ```
 
+A heredoc body may itself contain a line starting with ` ``` ` (e.g. a
+heredoc that writes out markdown) without being mistaken for the end of
+the code block:
+```sh {empty_output}
+$ cat > /tmp/md-exec-fence.txt << 'EOF'
+before
+```
+after
+EOF
+```
+
 ---
 
 ## Global Directives

scripts/md-exec.py

@@ -189,8 +189,13 @@ def parse(self):
 
             stripped = line.strip()
 
-            # Detect Code Snippets
-            if stripped.startswith("```"):
+            # Detect Code Snippets. Skip fence detection while inside an
+            # open heredoc body -- the heredoc content may legitimately
+            # contain a line starting with ``` (e.g. a heredoc that writes
+            # out markdown), and treating it as a fence boundary would cut
+            # the code block short mid-heredoc, leaving the shell command
+            # truncated and the heredoc unterminated.
+            if stripped.startswith("```") and heredoc_delimiter is None:
                 if in_block:
                     # End of block: save pending test
                     if current_cmd: