feat: add ygg skill install/uninstall for AI coding agents

[joch]

Summary

• Add ygg skill install / ygg skill uninstall commands that manage the ygg agent skill for Claude Code and Codex
• Skill file is embedded in the binary via go:embed so it works for Homebrew/binary installs
• Installs to ~/.claude/skills/ygg/ (Claude Code) and ~/.agents/skills/ygg/ (Codex)
• Shows "already installed, updating..." message when overwriting

Test plan

• Unit tests for install/uninstall logic (4 tests passing)
• ygg skill install creates skill files in both locations
• ygg skill install twice shows update message
• ygg skill uninstall removes both directories
• ygg skill uninstall when not installed shows info message
• Error propagation returns non-zero exit code on failure
• All existing tests still pass

[chatgpt-codex-connector]

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