Support external repository sources for plugins (don't require content to live in this repo)

## Problem

`marketplace.json` requires all plugin content to live inside this repo via relative paths. This forces plugin authors to duplicate content here rather than pointing to their canonical source repo.

## Use Case

We maintain [AzureCosmosDB/cosmosdb-agent-kit](https://github.com/AzureCosmosDB/cosmosdb-agent-kit) - 57+ Cosmos DB best-practice rules in the standard skill format (SKILL.md + AGENTS.md). The same skill is consumed by Claude Code, Gemini CLI, Cursor, and other agents via [Agent Skills](https://agentskills.io/). We want it discoverable through this registry without duplicating content that would drift from the source.

## Proposal

Support a `repository` field in `marketplace.json` that points to an external GitHub repo:

```json
{
"name": "cosmosdb-best-practices",
"repository": "AzureCosmosDB/cosmosdb-agent-kit",
"path": "skills/cosmosdb-best-practices",
"description": "Azure Cosmos DB best practices — 57+ rules for data modeling, partitioning, queries, SDK patterns, and more.",
"version": "1.0.0"
}
```

This is the same pattern used by npm, Homebrew, and the VS Code Marketplace - the registry indexes, the external repo provides content. The `workiq` plugin already does this for its MCP server (`npx @microsoft/workiq`), so the precedent exists for skills too.

## Happy to Help

We can submit a PR, help define the schema, and serve as a test case once this is supported.