docs(design): evaluate async runtime for SSH transport (#1411)

[oferchen]

Summary

• Evaluate runtime choice (tokio vs smol vs async-std) for future async SSH transport work, distinct from CI: remove unsupported windows aarch64 target #1593 which covers async I/O on the SSH path itself.
• Cross-reference closed work: Modularize CLI drive orchestration helpers #1779 (tokio dependency scope audit) and Decompose CLI filter rule handling #1780 (single-runtime invariant); both rule out a second runtime.
• Recommend tokio + russh (added in Refactor drive workflow into dedicated modules #1782); thrussh is deprecated, ssh2 is blocking. Defer migration behind --features embedded-ssh so the synchronous std::process SSH transport stays default.

Test plan

• Reviewer confirms the document is under 100 lines and scoped to runtime selection only.
• Reviewer confirms cross-references to CI: remove unsupported windows aarch64 target #1593, Modularize CLI drive orchestration helpers #1779, Decompose CLI filter rule handling #1780, and Refactor drive workflow into dedicated modules #1782 are accurate.
• No code paths touched; CI doc lints pass.