fix(skill_engine): preserve multiline frontmatter values

set_frontmatter_field wrote raw newlines into YAML, so parse_frontmatter
kept only the first line. Quote multiline scalars and drop old block
continuation lines when replacing a field.

.gitignore

@@ -51,6 +51,7 @@ tests/skill_engine/*
 !tests/skill_engine/test_evolver_length_recovery.py
 !tests/skill_engine/test_evolution_retry_idempotency.py
 !tests/skill_engine/test_analyzer_length_recovery.py
+!tests/skill_engine/test_set_frontmatter_multiline.py
 !tests/skill_engine/decision/
 tests/skill_engine/decision/*
 !tests/skill_engine/decision/test_analysis_adapter.py

openspace/skill_engine/skill_utils.py

@@ -58,10 +58,16 @@ _YAML_NEEDS_QUOTE_RE = re.compile(r"[:\#\[\]{}&*!|>'\"%@`]")
 
 def _yaml_quote(value: str) -> str:
     """Quote a YAML scalar value if it contains special characters."""
-    if not value or not _YAML_NEEDS_QUOTE_RE.search(value):
+    if not value:
         return value
-    escaped = value.replace("\\", "\\\\").replace('"', '\\"')
-    return f'"{escaped}"'
+    if "\n" in value or _YAML_NEEDS_QUOTE_RE.search(value):
+        escaped = (
+            value.replace("\\", "\\\\")
+            .replace('"', '\\"')
+            .replace("\n", "\\n")
+        )
+        return f'"{escaped}"'
+    return value
 
 
 def _yaml_unquote(value: str) -> str:
@@ -147,10 +153,17 @@ def set_frontmatter_field(content: str, field_name: str, value: str) -> str:
     new_line = f"{field_name}: {quoted}"
     found = False
     new_lines = []
+    skip_block = False
     for line in fm_text.split("\n"):
+        if skip_block:
+            if line.startswith((" ", "\t")) or line.strip() == "":
+                continue
+            skip_block = False
         if ":" in line and line.split(":", 1)[0].strip() == field_name:
             new_lines.append(new_line)
             found = True
+            # Drop old block-scalar / folded continuation lines under this key.
+            skip_block = True
         else:
             new_lines.append(line)
     if not found:

tests/skill_engine/test_set_frontmatter_multiline.py

@@ -0,0 +1,18 @@
+"""Regression: multiline frontmatter values must round-trip via parse_frontmatter."""
+
+from __future__ import annotations
+
+from openspace.skill_engine.skill_utils import parse_frontmatter, set_frontmatter_field
+
+
+def test_set_frontmatter_field_preserves_multiline_description():
+    out = set_frontmatter_field("---\nname: x\n---\n", "description", "line1\nline2")
+    assert parse_frontmatter(out)["description"] == "line1\nline2"
+
+
+def test_set_frontmatter_field_replaces_block_scalar_without_orphan_lines():
+    src = "---\nname: x\ndescription: |\n  old1\n  old2\n---\nbody\n"
+    out = set_frontmatter_field(src, "description", "new1\nnew2")
+    assert parse_frontmatter(out)["description"] == "new1\nnew2"
+    assert "old1" not in out
+    assert "old2" not in out