docs(rfcs): define OAuth and MCP gateway boundary

[f-trycua]

Summary

• define Cua Driver's standard, transport-neutral MCP boundary
• place OAuth, DCR, TLS, tunnels, and provider adapters in a separate optional gateway
• keep one canonical Driver tool inventory and require trusted local approval for remote desktop access

Why

PR #2063 demonstrates demand from remote MCP clients that require OAuth and HTTPS. Keeping the public OAuth edge inside cua-driver would couple the desktop runtime to token storage, tunnel operations, and client-specific compatibility rules.

This RFC moves those responsibilities to a separately owned gateway while keeping Driver as the final authorization point.

Refs #3197.

Salvaged from #2063. The RFC and commit preserve outdog-hwh's design credit; later implementations must preserve authorship for any material code they adapt.

Validation

• prettier --check rfcs/3197-oauth-mcp-gateway-boundary.md
• RFC template-section and placeholder scan
• git diff --check
• repository-relative target check for every linked source and RFC file

Review focus

• ownership and packaging of the gateway
• the minimum authenticated Driver connection-lease contract
• trusted local approval ownership on each platform
• first supported OAuth and MCP transport profiles
• DCR, token lifetime, revocation, storage, and tunnel policy

This is a draft decision document. It does not authorize implementation until the RFC is accepted.