docs(rfc): add named compute driver registry proposal

[elezar]

Summary

Add RFC 0005 proposing a named compute-driver registry for gateways. The proposal covers explicit sandbox placement with spec.compute_driver, named driver IDs, extension drivers, Docker endpoint configuration, router semantics, and scoped follow-up work for remote Docker and multi-Kubernetes auth.

Related Issue

No tracking issue. Related to #1703.

Changes

• Adds rfc/0005-named-compute-driver-registry/README.md.
• Documents the proposed registry config model, placement API, router behavior, telemetry treatment, and implementation phases.
• Captures open questions for remote Docker callbacks, driver health APIs, and multi-Kubernetes support.

Testing

• mise run pre-commit passes — not run: mise is not available on PATH in this shell.
• Unit tests added/updated — not applicable; RFC-only change.
• E2E tests added/updated — not applicable; RFC-only change.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)

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