Add optional privacy_policies array field to manifest.json that allows
developers to declare their data handling practices. This field accepts
an array of privacy policy statements that describe how the extension
collects, uses, and protects user data.
- Update manifest schema to include privacy_policies field
- Add empty privacy_policies array to all example manifests
- Document the new field in MANIFEST.md
- Replace manifest_version with $schema field pointing to official schema URL
- Support backward compatibility with legacy dxt_version field
- Update all examples and tests to use new $schema field
- Mark dxt_version as deprecated but still functional
- Require either $schema or dxt_version (at least one must be present)
