feat(skills): add llm-tool-best-practices reference from Anthropic + OpenAI official docs

skills/llm-tool-best-practices/SKILL.md

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+# SKILL.md — llm-tool-best-practices
+
+## Purpose
+Reference guide synthesizing official Anthropic and OpenAI best practices for building, reviewing, and importing agent skills/tools. Use this when:
+- Writing a new skill (SKILL.md or tool definition)
+- Reviewing a downloaded ClawHub skill for safety
+- Debugging why a tool is being misused or ignored by the model
+- Optimizing a skill for a specific LLM (Claude vs. GPT)
+
+## How to use
+Read `LLM_TOOL_BEST_PRACTICES.md` in this directory for the full reference.
+
+Key sections:
+1. **Universal principles** — applies to all LLMs
+2. **Anthropic/Claude-specific** — schema format, model selection, `input_examples`, eval approach
+3. **OpenAI/GPT-specific** — `strict` mode, tool count limits, reasoning model caveats, safety APIs
+4. **Differences summary** — quick comparison table
+5. **Skill safety checklist** — use before activating any downloaded skill
+6. **OpenClaw authoring notes** — how to write SKILL.md files for this workspace
+
+## Sources
+- https://platform.claude.com/docs/en/agents-and-tools/tool-use/implement-tool-use
+- https://www.anthropic.com/engineering/writing-tools-for-agents
+- https://platform.openai.com/docs/guides/function-calling
+- https://platform.openai.com/docs/guides/safety-best-practices
+
+## Last updated
+2026-03-05