commit-msg git hook

[schustafa]

Support for parsing a commit message that includes natural language like "Pairing with @mona" with a commit-msg hook. Automatically append the correct Co-authored-by: string (sourced from gh pairing-with) to the commit message.

Also adds:

• CI for the Ruby method that parses the commit message
• Scripted generation of the hook (so future updates to the parsing don't require copy/paste)