Docbench plugin foundation

Portable Agent Plugins 1.0 package for Docbench.

This first stage is intentionally skills-only. It packages the local-first file workflow and listing metadata without pretending that a ChatGPT file entrypoint or MCP server already exists.

The future MCP App/file-entrypoint work remains tracked in ../CHATGPT_PLUGIN.md. When a real host-integrated runtime exists, add mcp.json here and keep this package as the single plugin surface rather than creating a parallel wrapper.

The repository marketplace at .agents/plugins/marketplace.json exposes this folder for local authoring and testing.