Add Codex plugin packaging

[rgarcia]

Summary

• add a repo-scoped Codex marketplace for the existing Kernel skill bundles
• add Codex manifests and install metadata for kernel-cli, kernel-sdks, and generate-video
• document CLI and ChatGPT desktop installation

Testing

• validated each marketplace entry resolves to a plugin manifest and skill directory
• added the pushed branch as a Codex marketplace with --ref hypeship/add-codex-plugins
• installed all three plugins with Codex CLI and verified they were enabled with their bundled skills

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Note

Low Risk
Documentation and plugin packaging metadata only; no changes to skill content or runtime automation logic.

Overview
Adds Codex distribution for the existing Kernel skill bundles so they can be installed via Codex CLI or the ChatGPT desktop Plugins Directory.

Introduces .agents/plugins/marketplace.json with a Kernel marketplace that points at local paths for kernel-cli, kernel-sdks, and generate-video, each marked AVAILABLE with ON_INSTALL authentication. Each bundle gets a new .codex-plugin/plugin.json with display metadata, skill path (./skills/), capabilities, and default prompts.

The README gains a Codex section with codex plugin marketplace add / codex plugin add …@kernel commands and a note about installing from the desktop app after adding the marketplace.

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