Pull request runs only when the right paths change

In a monorepo, most pull requests never touch the site you test. Each one still spent a run and posted a check nobody needed. Now each repository decides.

• Path filters. Choose "Only when these paths change" and list one pattern per line, such as web/** or src/checkout/**. A bare folder counts as everything inside it. A pull request that changes none of them is skipped.
• Asking still works. Commenting @zeusqa run on a pull request runs it anyway, filters or not.
• Fails open. If GitHub cannot list the changed files, the run goes ahead instead of skipping silently.
• Set per repository. Filters live next to the rest of each repository's setup in Connections, GitHub.

A good first check for every preview is a smoke test.