fix(skill_engine): re-emit non-string frontmatter via first dump line

Avoid PyYAML document-end markers leaking into SKILL.md frontmatter
when normalizing dates and other non-string scalars.

openspace/skill_engine/skill_utils.py

@@ -66,23 +66,14 @@ def _yaml_quote(value: str) -> str:
 
 def _format_frontmatter_value(value: Any) -> str:
     """Serialize a frontmatter value for re-emit after PyYAML parse."""
-    if isinstance(value, bool):
-        return "true" if value else "false"
-    if value is None:
-        return "null"
-    if isinstance(value, (int, float)):
-        return str(value)
     if isinstance(value, str):
         return _yaml_quote(value)
-    try:
-        import yaml
+    import yaml
 
-        dumped = yaml.safe_dump(
-            value, default_flow_style=True, allow_unicode=True
-        ).strip()
-        return dumped
-    except Exception:
-        return _yaml_quote(str(value))
+    # First line only: scalar dumps include a trailing '...' document end.
+    return yaml.safe_dump(
+        value, default_flow_style=True, allow_unicode=True
+    ).splitlines()[0]
 
 
 def _yaml_unquote(value: str) -> str:
@@ -185,9 +176,9 @@ def normalize_frontmatter(content: str) -> str:
     """Re-serialize frontmatter with proper YAML quoting.
 
     Parses the existing frontmatter, then re-writes each value through
-    :func:`_yaml_quote` so that colons, hashes, and other special
-    characters are safely double-quoted.  The body after ``---`` is
-    preserved verbatim.
+    :func:`_format_frontmatter_value` so scalars (bool/int/date) and
+    strings with special characters re-emit safely.  The body after
+    ``---`` is preserved verbatim.
 
     Returns *content* unchanged if no frontmatter is found.
     """

tests/skill_engine/test_normalize_frontmatter.py

@@ -11,6 +11,15 @@ def test_normalize_frontmatter_bool_and_int_scalars() -> None:
     assert "enabled: true" in out
     assert "version: 2" in out
     assert "# body" in out
+    assert "..." not in out
+
+
+def test_normalize_frontmatter_date_scalar_no_document_end() -> None:
+    raw = "---\nname: demo\ncreated: 2024-01-01\nenabled: true\n---\n"
+    out = normalize_frontmatter(raw)
+    assert "created: 2024-01-01" in out
+    assert "..." not in out
+    assert "enabled: true" in out
 
 
 def test_normalize_frontmatter_string_still_quoted_when_needed() -> None: