Document marketplace distribution and Copilot Customize path

[Niels Laute (niels9001)]

Summary

• Add a public marketplace-distribution specification for GitHub Copilot, Claude Code, Cursor, and OpenAI.
• Explain how the GitHub Copilot app's new Customize tab discovers plugins from configured marketplaces and why updating the existing Awesome Copilot record is the path to appearing there.
• Separate compatibility, direct installation, public listing, and featured placement so the repository does not overstate distribution status.
• Define the shared release gate, package source for each client, submission/update process, validation steps, artwork requirements, and unresolved eligibility risks.
• Link the specification from the README and track execution in Track Agent Plugins 1.0 packaging and marketplace distribution #158.

Why

Agent Plugins 1.0 makes the skills portable, but it does not publish them to vendor catalogs. Each vendor has a different intake and update model:

• GitHub Customize reads configured plugin marketplaces; winui already has an Awesome Copilot entry that must be updated to the new package path and immutable release.
• Claude accepts public Git repository submissions through Anthropic's directory forms.
• Cursor manually reviews every version and currently documents a no-binaries policy that blocks the canonical package.
• OpenAI uses one universal Plugins Directory for Codex and ChatGPT and requires a generated review-safe skills bundle plus product/security guidance for local Windows execution.

The specification links directly to the official vendor documentation and marks undocumented behavior as unknown rather than guessing.

Validation

• Verified every local Markdown link resolves.
• Checked every external URL in the specification; public documentation links return successfully and authenticated submission portals return the expected access-controlled response.
• Reviewed the document against the repository's current staging package layout and release policy.

Documentation-only change; no changelog entry is required by CONTRIBUTING.md.

Preview

Read the rendered marketplace distribution specification.