Create Onboarding Guide

Produce one self-contained HTML file that takes a newcomer from zero to "I know where things live and how the main flows run" in one sitting.

1. Resolve the output directory

Same per-installer configuration as this plugin's other guide skills:

1. ${CLAUDE_PLUGIN_ROOT}/config.json → outputDir (shared across this plugin's skills)
2. config.json next to this SKILL.md → outputDir (skill-level override)
3. Neither exists → read config.example.json for the shape, ask the user for a real absolute path, then write the skill-level config.json (git-ignored).

Honor openAfterGenerate (default true).

2. Confirm the target