Shared path buckets for bucketed pipelines #146

Repositories run every suite on every pull request, whatever the diff touches. A shared path-bucket mechanism lets a repository declare which paths reach which suites, and gate its jobs on what a pull request's diff actually reaches.

Scope

• A script that reads a repository's .github/buckets.yml, self-tests it against the fixtures it declares, and outputs which buckets and groups a diff reaches.
• A composite action and a reusable workflow (path-buckets.yml) any repository can call as its changes job.
• Documentation of the file format and the pipeline shape.

Follow-up

JorisJonkers-dev/identity adopts it once it is released.