docs: add install conflict troubleshooting section

[dtsong]

Goal

Add a concise troubleshooting section to README.md that explains what to do when ./install.sh reports path conflicts (for example, existing ~/.config/opencode/skills).

Why

First-time contributors can get blocked by conflict errors and may not know when to use --conflict-policy skip vs cleanup.

Acceptance Criteria

• Add a short "Install conflict troubleshooting" subsection in README.md
• Include at least one concrete conflict example and recommended resolution path
• Include when --conflict-policy skip is safe to use
• Keep wording beginner-friendly and copy/paste oriented