feat(skill): refactor skill system with router + references structure

[Citrus086]

Summary

• Add router-based SKILL.md with trigger keywords for each category
• Split detailed docs into references/ directory for better organization
• Update _install_skill() to copy entire skill directory with references

Changes

SKILL.md (Router)

• Add routing table for quick navigation
• Add rich trigger keywords (e.g., xiaohongshu/xhs/小红书/红书)
• Include zero-config quick commands

references/ directory

• search.md — Exa AI search
• social.md — XHS, Douyin, Twitter, Weibo, Bilibili, V2EX, Reddit
• career.md — LinkedIn
• dev.md — GitHub CLI
• web.md — Jina Reader, WeChat Articles, RSS
• video.md — YouTube, Bilibili, Podcast transcription

CLI

• _install_skill() now copies entire skill directory including references/

Benefits

1. Better organization — Main SKILL.md stays concise, details in references
2. Richer triggers — More keywords for better intent matching
3. Easier maintenance — Each category in its own file
4. Backward compatible — Install command works the same way

Test plan

• Run agent-reach install — verify skill files are copied correctly
• Check ~/.agents/skills/agent-reach/ structure
• Verify references/ directory exists with all 6 files

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