Add a local skill marketplace

[ecraig12345]

Set up the repo as a "marketplace" so the change files skill can be more conveniently installed and updated. (Very experimental and subject to change.)

# Claude Code
/plugin marketplace add microsoft/beachball
/plugin install beachball-change-file@beachball-plugins
# To update later:
/plugin marketplace update beachball-plugins

# Copilot CLI
copilot plugin marketplace add microsoft/beachball
copilot plugin install beachball-change-file@beachball-plugins
# To update later:
copilot plugin update beachball-change-file

To support the marketplace structure without duplication, .claude/skills/beachball-change-file/SKILL.md is now a symlink to .claude-plugin/plugins/beachball-change-file/skills/beachball-change-file/SKILL.md. I also added a lint script to ensure skill versions stay in sync across multiple locations.