RFC 0005: Sandbox proxy egress adapter model

[johntmyers]

Summary

Drafts RFC 0005 for reshaping sandbox proxy egress around transport adapters, shared EgressIntent / EgressDecision authorization, and relay-owned byte handling. The proposal covers CONNECT, forward HTTP, DNS, local services, native TCP, supervisor middleware, credential injection, WebSocket/body injection, JSON-RPC, MCP, and optional process identity for sidecar-style modes.

Related Issue

N/A - RFC proposal.

Changes

• Adds RFC 0005 for the sandbox proxy egress adapter model.
• Splits current-state notes, technical details, and implementation plan into RFC appendix documents.
• Documents flow diagrams for unified adapters, relay handling, DNS/native TCP, local services, and deployment modes.
• Accounts for recent networking changes including nftables, supervisor middleware, JSON-RPC/MCP L7 support, and identity-less runtime modes.

Testing

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

RFC-only documentation change. Ran git diff --check and ASCII scans on the RFC files. Pre-commit was intentionally not run for this RFC draft.

Checklist

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