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.

まず marketplace.json から始め、既存の workiq プラグインが外部 MCP サーバーをどのように表現しているかを調べます。外部 skill コンテンツの repository と path をどのように扱うべきかを定義し、そのうえで Cosmos DB の例をこの repository にコンテンツを重複させずにインデックス化できることを確認します。

索引モデルが issue の本文から書いたものです。