docs: multi-host computer_use design (blocked on Cua paired hosts)

[skyzea1]

Summary

• Add docs/design/71157-multi-host-computer-use.md — Hermes as thin consumer of Cua paired multi-host Driver
• Document current one machine per process limitation in computer-use user docs, with workaround + links
• No runtime / tool schema change in this PR (cannot implement remote host attach until Cua ships protocol)

Why docs PR (not implementation)

Remote desktop control protocol + pairing must live in trycua/cua. Implementing Hermes-only remote click would be the wrong layer and likely rejected.

Upstream:

• Issue: [RFC] Self-hosted multi-host paired remote Driver for physical PCs (comp1→comp2/comp3) trycua/cua#2562
• RFC PR: docs: propose RFC 2562 self-hosted paired multi-host Driver trycua/cua#2563

Hermes tracking: #71157

Related (orthogonal)

• feat: add config option to connect computer_use to existing cua-driver daemon (fix bundle_identity warning) #65259 — connect to local existing cua-driver daemon
• [Feature]: First-class PC-to-PC peer channel between two hermes instances](https://github.com/NousResearch/hermes-agent/issues/new?title=[Feature]: First-class PC-to-PC peer channel between two hermes instances) #69147 — Hermes↔Hermes peer messaging (not Driver multi-host)

Test plan

• Docs-only; no code paths changed
• Docs preview / link check as CI requires
• Maintainer: confirm design note location (docs/design/) is acceptable

Ask

If Cua accepts paired multi-host Driver, is a follow-up default-off consumer implementation PR welcome under existing computer_use (host arg / session pin + approvals), without a new core tool?

[OutThisLife]

Pointer for whenever the upstream block clears: #90380 adds a ComputerUseProvider ABC and registry, so multi-host computer use is now a provider question rather than a core-dispatcher one. A paired-host runtime registers through ctx.register_computer_use_provider() and is selected by computer_use.provider in config.yaml.

Worth a pass over the design doc against that seam before this unblocks — some of what it describes as needing core changes no longer does.