chore: add public JSON schema for directory-entry.json

[rbadillap]

This PR adds a new JSON Schema for the DirectoryEntry type, useful for external directories or validate registries purposes, I'm following the same pattern as registry.json and registry-item.json

The schema validates:

• Required fields: name, homepage, url, description, logo
• Validates name matches namespace pattern: @[a-zA-Z0-9][a-zA-Z0-9-_]*
• Validates url includes {name} placeholder
• All fields are required and properly typed

[vercel]

@rbadillap is attempting to deploy a commit to the shadcn-pro Team on Vercel.

A member of the Team first needs to authorize it.