refactor skills loader markdown metadata parsing

[nayihz]

📝 Description

This PR is primarily intended to fix the startup failure where picoclaw reports invalid skill from workspace while loading workspace skills.

To address that, it refactors pkg/skills/loader.go metadata extraction for SKILL.md files:

• Parse markdown body via gomarkdown AST and extract metadata from first H1 + first paragraph.
• Parse frontmatter once (splitFrontmatter) and reuse body/frontmatter slices instead of scanning twice.
• Replace ad-hoc YAML parsing with yaml.v3 for robust multiline/structured frontmatter support.
• Add/extend unit tests for markdown metadata extraction, multiline YAML description, heading fallback, and HTML comment tolerance.

🗣️ Type of Change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 📖 Documentation update
• ⚡ Code refactoring (no functional changes, no api changes)

🤖 AI Code Generation

• 🤖 Fully AI-generated (100% AI, 0% Human)
• 🛠️ Mostly AI-generated (AI draft, Human verified/modified)
• 👨‍💻 Mostly Human-written (Human lead, AI assisted or none)

🔗 Related Issue

N/A

📚 Technical Context (Skip for Docs)

• Reference URL: https://github.com/gomarkdown/markdown
• Reasoning: The key goal is to make skill metadata parsing tolerant to valid markdown variants (including comment blocks and richer frontmatter formatting) so workspace skills are not incorrectly rejected as invalid during process startup.
• Binary size impact: This PR introduces a new direct dependency on github.com/gomarkdown/markdown. Based on local darwin/arm64 builds, binary size changed from 20,815,442 bytes to 21,240,146 bytes, i.e. +424,704 bytes (+2.04%).

🧪 Test Environment

• Hardware: MacBook (local dev machine)
• OS: macOS
• Model/Provider: N/A (unit tests only)
• Channels: N/A

📸 Evidence (Optional)

Click to view Logs/Screenshots

go test ./pkg/skills -run "TestGetSkillMetadata|Test(ExtractFrontmatter|StripFrontmatter|ListSkills|SkillRoots)" -count=1

Binary size check:

• Before: -rwxr-xr-x ... 20815442 ... picoclaw-darwin-arm64
• After: -rwxr-xr-x ... 21240146 ... picoclaw-darwin-arm64

☑️ Checklist

• My code/docs follow the style of this project.
• I have performed a self-review of my own changes.
• I have updated the documentation accordingly.

[CLAassistant]

All committers have signed the CLA.

[yinwm]

LGTM! This is a solid refactor that improves metadata parsing robustness:

• Standard yaml.v3 parser replaces ad-hoc YAML parsing
• Markdown AST parsing is more accurate than regex
• Backward compatibility preserved (JSON → YAML → Markdown → fallback)
• Good test coverage for new functionality

Minor suggestion for follow-up: update the stale comments in parseSimpleYAML and add cross-platform newline tests.