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.

Issue 中没有指明仓库文件、测试或实现入口。开始前请明确实现范围；完成意味着为 push rulesets 提供基于路径的排除，并在设置时进行验证和选择性强制执行。

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