fix: skill description not fully loaded from SKILL.md frontmatter

[1thanMoon]

Problem

_parse_frontmatter splits lines manually and only reads key: value pairs.
When a skill's SKILL.md uses YAML block scalars (|), the description
is parsed as "|" — all indented continuation lines are silently dropped.

Root Cause

# Before: only handles simple key: value
for line in match.group(1).strip().splitlines():
    if ":" in line:
        key, val = line.split(":", 1)

Fix

Replace manual parsing with yaml.safe_load(), which correctly handles
all standard YAML syntax including multi-line block scalars.

Test

Skills with description: | now load their full description correctly.
For example, agent-builder's full description is now visible in the system prompt
instead of a single | character.

[vercel]

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