[DOCS + FEATURE] Cross-marketplace plugin dependencies: docs cite an allowlist but never define it

Summary

plugin.json dependencies can include a marketplace field to reference
a plugin in another marketplace. The docs say an allowlist gates this
feature but never document it. Users cannot declare cross-marketplace
dependencies.

The documentation gap

From plugin-dependencies.md,
the marketplace field reads:

A different marketplace to resolve name in. Cross-marketplace dependencies are blocked unless the target marketplace is allowlisted in the root marketplace's marketplace.json.

Neither plugin-marketplaces.md
nor the plugins reference
specifies:

• The allowlist's JSON field name in marketplace.json
• The allowlist entry schema (bare name? owner/repo? source object?)
• A worked example
• Whether the mechanism is implemented, stubbed, or admin-only

strictKnownMarketplaces is managed-settings-only and controls which
marketplaces users can add — unrelated to dependency resolution.

Real-world impact

I maintain a private curated marketplace. One of its embedded plugins
needed to extend agents from pr-review-toolkit in
anthropics/claude-plugins-official. Declaring
{"name": "pr-review-toolkit"} failed with
pr-review-toolkit@<my-marketplace> not installed — the resolver
scoped to my own marketplace. I found no documented way to declare a
cross-marketplace dependency.

Workaround: I copied the agent files verbatim under Apache-2.0
attribution, with per-file blob-SHA tracking and drift-detection tooling.
A usable marketplace field would eliminate this overhead.

Requested resolution

One or both of:

1. DOCS: Specify the allowlist's field name, schema, and an example

on plugin-marketplaces.md, linked from plugin-dependencies.md.

1. FEATURE: If the allowlist is not production-ready, add a

requiredMarketplaces field to marketplace.json that prompts users
to add any missing companion marketplace at install time.

Related

• #26122 — Cross-marketplace plugin references (stale-closed with no

Anthropic engagement)

• #48864 — [DOCS] Plugin docs missing plugin.json dependency

declarations and auto-install behavior

• #49311 — [DOCS] Plugins reference missing dependency version

requirement and recovery docs