Linear PR Check

Simple action that checks that a PR contains a Linear issue linked through Linear's supported mechanisms.

Linear supports four ways to link issues with your pull requests:

1. Include *issue ID* in the branch name
2. Include *issue ID* in the PR title
3. Include *issue ID* with a magic word in the PR description (e.g., Fixes ENG-123) similar to GitHub Issues
4. Include the issue URL in the PR description (e.g., https://linear.app/yourteam/issue/ENG-123/issue-title)

Using it

TBD