fix(skill_engine): preserve multiline frontmatter values

[santhreal]

Problem

set_frontmatter_field wrote raw newlines into YAML, so parse_frontmatter kept only the first line. Replacing a | block also left orphan indented lines.

Repro

from openspace.skill_engine.skill_utils import set_frontmatter_field, parse_frontmatter

out = set_frontmatter_field("---\nname: x\n---\n", "description", "line1\nline2")
assert parse_frontmatter(out)["description"] == "line1\nline2"  # was "line1"

Fix

Quote multiline scalars with escaped \n, unescape them in the no-PyYAML fallback, and drop old block continuation lines when replacing a field.