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/sandbox-isolation.md

@@ -222,7 +222,7 @@ Triggered automatically if no credentials found. Use `/login` inside Claude Code
 ### Limitations
 
 - **macOS and Windows only** for microVM mode. Linux uses legacy container-based sandboxes (Docker Desktop 4.57+).
-- **Docker Desktop required** — not available with standalone Docker Engine.
+- **Docker Desktop required** — not available with standalone Docker Engine. Community alternatives like [dclaude](https://github.com/jedi4ever/dclaude) (Patrick Debois) wrap Claude Code in standard Docker containers for Docker Engine-only environments, but use container isolation (not microVM) and mount the host Docker socket — weaker security boundary.
 - **MCP Gateway not yet supported** inside sandboxes.
 - **No GPU passthrough** — not suitable for ML training workloads.
 - **Workspace sync is one-way**: changes inside the sandbox propagate to the host, but concurrent host edits may conflict.