release: v3.20.6 - agentskills.io integration + 4 resource evaluations

- agentskills.io open standard: frontmatter table, skills-ref CLI, portability section
- Agent Skills supply chain risks (security-hardening.md §1.2)
- anthropics/skills (60K+★) added to complementary resources
- 16 new reference.yaml entries
- Resource evaluations: agentskills.io (4/5), Skill Doctor (2/5), dclaude (new), paddo (new)
- Sandbox isolation + README updates

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

guide/ai-ecosystem.md

@@ -1873,6 +1873,10 @@ An **on-machine AI coding agent** developed by Block (formerly Square), released
 - You value Claude's specific reasoning capabilities and can't substitute
 - You don't want to manage LLM API credentials
 
+### Skill Portability
+
+Both Claude Code and Goose support the [Agent Skills open standard](https://agentskills.io) (agentskills.io). Skills you create with SKILL.md are portable across 26+ platforms including Cursor, VS Code, GitHub, OpenAI Codex, and Gemini CLI. Claude Code-specific fields (`context`, `agent`) are ignored by other platforms but don't break compatibility.
+
 ### Trade-offs
 
 | Goose Advantage | Goose Limitation |