docs(rfc): propose repository migration for a smaller core

[drew]

Summary

Propose splitting OpenShell into a smaller core and focused repositories so new maintainers can own platforms, packaging, SDKs, and provider integrations. Keep RFC 0019 short and conversational for early discussion.

Related Issue

Closes #4425

Changes

• Define the proposed repository responsibilities and ownership expectations.
• Outline an incremental migration, risks, and alternatives.
• Leave macOS isolation, microVM placement, host process assembly, and cross-repository compatibility as open questions.

Testing

• Markdown lint passes (mise exec -- markdownlint-cli2 rfc/0019-repository-migration/README.md).
• git diff --check passes.
• Unit and E2E tests are not applicable to this RFC-only change.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture documentation updates are not applicable to this draft proposal.

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