[FEATURE] Support community/external plugins in marketplace.json — AgentFlow as first candidate

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

The current marketplace.json only supports bundled plugins with local source paths (./plugins/...). There's no mechanism for the community to contribute plugins that live in external repositories. This means valuable community-built plugins like task orchestrators, SDLC pipelines, and specialized workflows can't be discovered through the official plugin marketplace.

I built AgentFlow (https://github.com/UrRhb/agentflow), an autonomous dev pipeline that uses your Kanban board as the orchestration layer for Claude Code. It already has a .claude-plugin directory structure but can't be listed in the marketplace because external sources aren't supported.

Proposed Solution

Add support for external plugin sources in marketplace.json. Currently all plugins use "source": "./plugins/..." (local paths). Adding support for GitHub URLs like "source": "https://github.com/UrRhb/agentflow" would allow community plugins to be discoverable via claude plugin install.

As a first community plugin candidate, AgentFlow provides:

• Autonomous SDLC pipeline orchestrated through your Kanban board (Asana, GitHub Projects, Linear)
• - 7-stage pipeline: Backlog → Research → Build → Review → Test → Integrate → Done
• - - Stateless crontab orchestrator (crash-proof), deterministic quality gates, adversarial AI review
• - - - Per-task cost tracking with automatic guardrails
• - - - - Already structured as a Claude Code plugin with a .claude-plugin directory

Alternative Solutions

Currently users manually copy plugin files into their projects. A separate community marketplace repo (like awesome-claude-code) could also work, but native integration via the official marketplace would provide better discoverability and a standardized installation experience.

Priority

Medium - Would be very helpful

Feature Category

Developer tools/SDK

Use Case Example

1. I build an autonomous dev pipeline as a Claude Code plugin (AgentFlow)
2. 2. I structure it with a .claude-plugin directory following the official format
3. 3. I want users to discover it via claude plugin search agentflow
4. 4. I want users to install it via claude plugin install agentflow
5. 5. Currently this isn't possible because marketplace.json only supports bundled local plugins
6. 6. With external source support, community plugins could be installed the same way as official ones

Additional Context

AgentFlow repo: https://github.com/UrRhb/agentflow

The repo already includes a .claude-plugin directory structure and Claude Code skills (/spec-to-board, /sdlc-orchestrate, /sdlc-worker, /sdlc-health, /sdlc-stop). It demonstrates a realistic community plugin use case for the marketplace.