Add install script for one-command setup

[TheStack-ai]

Context

Currently users must manually copy skill folders to ~/.claude/skills/. There's no automated install.

What to do

Add an install.sh that:

1. Copies all skills from skills/ to ~/.claude/skills/
2. Skips already-installed skills (or asks to overwrite)
3. Prints which skills were installed

Also add to README: curl -fsSL https://raw.githubusercontent.com/TheStack-ai/swing-skills/main/install.sh | bash

Difficulty: Easy