docs: GenBox Push extension integration proposal

[liwei9745]

Summary

Structured proposal to extend chatgpt2api so generated images can be forwarded to
a GenBox media library over the documented GenBox Push contract (v1).

This PR is documentation only: no sender code, no configuration, no
credentials, and no environment-specific values. It follows the project's
documentation style by linking the proposal from the README experimental section.
The existing docs/feature-status.en.md is unchanged.

What is included

• docs/genbox-push-extension-proposal.md: goal and non-goals, Push contract
  v1 summary, three narrow task boundaries (Push Foundation, Generation and
  Gallery UI, Scheduler and Confirmed Cleanup), suggested data model and item
  states, acceptance-test matrix, security and privacy constraints,
  compatibility and migration notes, and a proceed path.
• README.md: one link line under the experimental section.

Reference contracts

GenBox receiver contract details live in the public GenBox repository
(liwei9745/GenBox): docs/INTEGRATION.md,
docs/chatgpt2api-push-integration.md, and
docs/UPSTREAM-VIBE-CODING-GUIDE.md.

Notes

• This is an integration proposal; it does not claim sender-side code exists.
• Push direction is additive; unchanged behaviour when no GenBox destination is configured.
• No secrets, ports, deployment addresses, or environment-specific values are included.