Path-based exclusions for push rules in Rulesets [Public Preview]

### Value Prop
Path-based exclusions in push rulesets let you apply file push rules only where they matter — protecting critical paths while skipping generated code, documentation, or vendored assets that don't need the same governance. You get precise, surgical control over enforcement without loosening protections elsewhere in your repository. Exclusions are validated at setup time, so misconfigurations are caught before they reach your developers.

### Expected Outcome
Teams can enforce strong push rules across their repositories without creating unnecessary friction for paths that follow different conventions. By giving admins the flexibility to exclude specific directories, this feature reduces workarounds, support requests, and the pressure to choose between blanket enforcement and blanket exemption. The result is a more accurate and trustworthy governance layer that reflects how real codebases are structured.

No repository files, tests, or implementation entry points are named in the issue. Clarify the implementation area before starting; done means path-based exclusions for push rulesets, with setup-time validation and selective enforcement.

索引モデルが issue の本文から書いたものです。