[FEATURE] Support marketplace references inside marketplace.json

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

We are building an internal plugin marketplace and want plugin setup to be as frictionless as possible for our users. Ideally, users should only need to add our internal marketplace and install from there.
The problem is that some plugins or capabilities may live in other marketplaces, for example the https://github.com/anthropics/skills marketplace or other external marketplaces that are not automatically installed by Claude Code. In the current workflow, this means we have to instruct users to install multiple marketplaces manually. That adds setup overhead, creates confusion, and makes enterprise rollout harder to manage.
From an organization perspective, we want to provide one curated entry point for users.

Proposed Solution

Allow marketplace.json to reference other marketplaces.
The ideal user experience would be:
A user adds or installs our internal marketplace
That marketplace declares references to one or more additional marketplaces
Claude Code automatically resolves those referenced marketplaces during /plugin install
The user gets access to all curated plugins through a single installation flow
This would let organizations expose one internal marketplace as the single entry point, while still depending on selected external marketplaces behind the scenes.
It would also be helpful if Claude Code handled duplicate marketplace entries or naming conflicts gracefully when resolving referenced marketplaces.

Alternative Solutions

The current workaround is to document additional manual installation steps and ask users to install multiple marketplaces themselves.
That works technically, but it increases friction and weakens the value of having a curated internal marketplace. It also creates more support effort, because users need to understand which extra marketplaces are required and install them in the correct way.

Priority

High - Significant impact on productivity

Feature Category

Configuration and settings

Use Case Example

Example scenario:
We provide an internal plugin marketplace for developers in our company
Users should only need to configure this single internal marketplace
Some curated plugins in our marketplace depend on content or plugins from another marketplace, such as https://github.com/anthropics/skills
Today, we must additionally tell users to manually install those other marketplaces
With this feature, users would run one /plugin install flow against our internal marketplace
Claude Code would automatically resolve the referenced marketplaces
This would reduce setup friction, simplify onboarding, and make internal rollout much easier

Additional Context

This is especially useful for enterprise environments that want to provide a curated and controlled developer experience.
Instead of exposing users directly to multiple public or third-party marketplaces, the organization could offer one approved marketplace that aggregates internal and external sources.
In practice, this would make marketplace composition much cleaner and would support a more scalable internal plugin ecosystem.