docs: improve ONBOARDING.md with practical examples and visual aids

[q-soriarty]

Problem

ONBOARDING.md provides a solid structural overview but lacks:

• Concrete example walkthrough — a step-by-step example of contributing a real change (from issue to merged PR) with actual commands
• Common mistakes for newcomers — what new contributors typically get wrong in their first PR
• Visual overview — a diagram showing how SKILL.md fits into the Claude Code ecosystem (skill directory → Claude Code session → activated on git keywords)
• FAQ — answers to questions new contributors commonly ask ("Can I use this with other AI assistants?", "What if my project doesn't use Gitflow?", "Do I need all files or just SKILL.md?")

Expected behavior

Expand ONBOARDING.md with at least:

1. A worked example of a complete contribution cycle
2. A short FAQ section

[github-actions]

Closed by #86 (merged into develop).