fix(skill_engine): read block-scalar values in get_frontmatter_field

[santhreal]

Problem

get_frontmatter_field returned the literal | for description: | block scalars, so callers skipped filling a real description.

Repro

from openspace.skill_engine.skill_utils import get_frontmatter_field

src = "---\nname: x\ndescription: |\n  line1\n  line2\n---\n"
assert get_frontmatter_field(src, "description") == "line1\nline2"

Fix

Collect indented continuation lines for | / > markers in the existing line walker (works without PyYAML; plain scalars unchanged).