Manage vulnerability noise at scale with auto-dismiss policies

Introduces auto-dismiss vulnerability policies that allow users to define rules in a YAML file to automatically dismiss findings based on file path, directory, or vulnerability identifier. Provides ready-to-use configurations for dismissing vulnerabilities in test code, vendored/third-party code, known false positive CVEs, generated code, infrastructure-mitigated vulnerabilities, and CVE families. Details the process of defining, merging, and activating these policies, and how they are applied during default-branch pipelines. Explains the criteria types (file_path, directory, identifier) and dismissal reasons.