add --skill flag and include claude skill in chad init

[cs01]

Summary

• Add chad --skill flag that prints a Claude Code SKILL.md to stdout (for existing projects)
• chad init now also creates .claude/skills/chadscript/SKILL.md (for new projects)
• Skill content lives in lib/skill.md, embedded at compile time in the native binary via ChadScript.embedFile()
• Skill covers CLI usage, supported TS features, built-ins, file embedding, and key differences from Node.js

Test plan

• chad --skill prints skill content to stdout
• chad --skill > file.md redirect works
• chad init creates .claude/skills/chadscript/SKILL.md with correct directory structure
• chad init skips skill file if already exists
• chad --help shows --skill flag
• all compiler tests pass