Add git-subdir source type support to plugin marketplace schema

### Describe the feature or problem you'd like to solve

/plugin marketplace add anthropics/claude-plugins-official fails with ~40 plugins.N.source: Invalid input errors because Copilot CLI's marketplace schema doesn't accept the git-subdir source type used widely in the Claude Code plugin ecosystem.

### Proposed solution

_No response_

### Example prompts or workflows

Example of a rejected entry (valid in Claude Code, rejected by Copilot CLI):

{
"name": "stripe",
"source": {
"source": "git-subdir",
"url": "https://github.com/stripe/ai.git",
"path": "providers/claude/plugin",
"ref": "main",
"sha": "ec93d4c4b9ffdbc994ac45ce692d4ec1cdb755f0"
}
}

Currently only string paths and {"source": "url", "url": "...", "sha": "..."} (whole-repo) are accepted. Without git-subdir support, ~40 of ~170 plugins in Anthropic's
official marketplace — including AWS, Stripe, Zapier, Shopify, Microsoft Dataverse, SAP, Auth0, Airtable, Expo, Railway, Neon, Snowflake, ServiceNow, and others — cannot be
installed via Copilot CLI. Please add git-subdir to the accepted source schema for plugin marketplace entries so the Claude Code ecosystem works out of the box.

Repro: /plugin marketplace add anthropics/claude-plugins-official

### Additional context

_No response_

`/plugin marketplace add anthropics/claude-plugins-official` のエントリーポイントから始め、マーケットプレイスのソース検証またはスキーマを特定します。`git-subdir` が拒否されることを再現し、その後、`source`、`url`、`path`、`ref`、`sha` を含むエントリが受け入れられ、マーケットプレイスが正常にインストールされ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。