Allow built-in Agent Plugin Marketplaces to be blocked

Describe the feature or problem you'd like to solve

copilot-plugins and awesome-copilot marketplaces always show and cannot be removed. We have our own universal internal Agent Plugin Marketplace. We'd like the option to block and hide any Copilot marketplaces.

Proposed solution

We maintain our own internal Enterprise wide Agent Plugin Marketplace, run as an innersource model. We have strong guardrails and guidelines in place to ensure skills do not overlap, follow consistent naming, structural and content conventions. They are also crafted specifically to our tools, languages, systems, & processes, etc.

We curate and clone external plugins/skills into this internal marketplace as we need them, tracking provenance so we can get updates, new features, etc. in a controlled and reviewed manner. This is especially important as supply chain attacks via prompt injection are on the rise and we do want users installing unreviewed external context enrichment.

Currently strictKnownMarketplaces doe not apply to copilot-plugins and awesome-copilot Agent Plugin Marketplaces. We'd like the ability to also have these, and any future integrated marketplaces, blocked and hidden.

Example prompts or workflows

No response

Additional context

まず、strictKnownMarketplaces 設定がどのように処理されているか、また組み込みの copilot-plugins および awesome-copilot マーケットプレースがどこで登録・表示されているかを追跡します。これらのマーケットプレース（将来の統合マーケットプレースを含む）をブロックおよび非表示にする動作を定義し、内部マーケットプレースが引き続き使用可能であることを検証します。issue では具体的なファイルやテストは指定されていません。