Extension registry should support a git URL field for source code links

Problem

When users discover community extensions in the swamp extension registry, there is no way to find the source code for those extensions. Users may want to review the source before trusting an extension, contribute improvements, or fork an extension as a starting point for their own.

Proposed Solution

Add an optional git (or repository) field to the extension registry schema that allows extension authors to link to their source code repository.

Changes would be needed in:

• Extension registry schema to accept the new field
• Extension type metadata (e.g. in model.type exports or a companion manifest)
• swamp model type describe output to display the repository URL when present
• swamp model type search could optionally show the URL in results

Example usage in an extension:

export const model = {
  type: "@bixu/honeycomb",
  version: "2026.03.02.16",
  repository: "https://github.com/bixu/swamp",
  // ...
};