fix: resolve inline backticks in code fences breaking HTML comment stripping

The fence-matching regex treated inline ``` characters on a content line
as the closing fence delimiter, causing HTML comments after the inline
backticks (but still inside the real fence) to be incorrectly stripped
from the prompt. The fix requires the closing fence backreference to be
preceded by a newline, matching CommonMark's rule that closing code fences
must start on their own line.

Co-authored-by: Ralphify <noreply@ralphify.co>

Author: 2 people

src/ralphify/_frontmatter.py

@@ -55,8 +55,12 @@
 # Backreferences (\2 / \3) ensure the closing fence has the same number
 # of characters as the opening, so a ```` fence is not broken by an
 # inner ``` — the inner ``` is treated as content, not a fence boundary.
+# The ``\n`` before each backreference requires the closing fence to
+# start on a new line, preventing inline backticks/tildes from being
+# mistaken for a closing fence (matching CommonMark's rule that closing
+# code fences must be on their own line).
 _FENCE_OR_COMMENT_RE = re.compile(
-    r"((`{3,}).*?\2|(~{3,}).*?\3)|<!--.*?-->",
+    r"((`{3,}).*?\n[ \t]*\2|(~{3,}).*?\n[ \t]*\3)|<!--.*?-->",
     re.DOTALL,
 )
 

tests/test_frontmatter.py

@@ -151,6 +151,23 @@ def test_html_comment_inside_four_backtick_fence_preserved(self):
         assert "<!-- keep this comment -->" in body
         assert "<!-- strip this comment -->" not in body
 
+    def test_html_comment_after_inline_backticks_inside_fence_preserved(self):
+        """When a ``` fence contains inline ``` characters on a content
+        line, the inline backticks must NOT be treated as the closing
+        fence.  HTML comments that follow the inline backticks but are
+        still inside the real fence must be preserved."""
+        text = (
+            "---\nagent: claude\n---\n"
+            "```\n"
+            "code with ``` inline\n"
+            "<!-- keep this -->\n"
+            "```\n"
+            "<!-- strip this -->"
+        )
+        _, body = parse_frontmatter(text)
+        assert "<!-- keep this -->" in body
+        assert "<!-- strip this -->" not in body
+
     def test_invalid_yaml_raises_value_error(self):
         text = "---\n: invalid: yaml: [unclosed\n---\nBody"
         with pytest.raises(ValueError, match="Invalid YAML"):