docs(rfc): add core architecture RFC

[drew]

Summary

Add RFC 0001 for the proposed core OpenShell architecture and move it into the repo's rfc/ directory for review.

Related Issue

Closes OS-52

Changes

• add rfc/0001-core-architecture.md from the current Linear draft
• adapt the document to the repo RFC format with YAML front matter
• capture the current architecture direction, including driver loading, driver versioning, published artifacts, and the split between control-plane identity and sandbox identity
• document the current mTLS gateway auth path as a control-plane identity option

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Ran mise run pre-commit; the repo-level test:rust task failed in the existing workspace test suite. The RFC itself is a markdown-only change.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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