docs(rfc): propose multiple openYuanRong SDK backends

[WenYuLuo]

Summary

Add Draft RFC 0001 for supporting two optional openYuanRong Python
backends behind the stable akernel_sdk.Sandbox API:

• openyuanrong-sdk, preserving the current actor-based implementation;
• openyuanrong-sandbox, using the frontend sandbox v1 and RRT path.

Proposed decisions

• Use openyuanrong-sdk and openyuanrong-sandbox consistently as the
  dependency extra names, runtime backend identifiers, and diagnostic names.
• Keep openyuanrong-sdk as the compatibility default when both backends are
  installed.
• Move backend-specific command and filesystem behavior behind internal
  adapters while preserving AKernel public value types.
• Validate backend capabilities before remote creation and never
  automatically retry creation with another backend.
• Treat current HTTP/RRT gaps such as Kata, S3 rootfs, node placement,
  create-time cwd, and name-based deletion as explicit capability boundaries.

This PR contains design documentation only. It does not change SDK behavior or
deployment configuration.

Validation

• git diff --cached --check
• 57 SDK unit tests passed with the repository-local Python 3.12 environment
• Ruff passed
• mypy passed
• Markdown fence, final-newline, and trailing-whitespace checks passed

make sdk-test cannot enter the test target on this macOS checkout because
the Makefile selects /usr/bin/bash, which is absent. The underlying unittest
command from that target passed directly.

[WenYuLuo]

This Draft PR is superseded by RFC discussion in #10. The proposal has been moved to the Issue so design decisions can be discussed before any repository document or implementation is merged.