refactor: share control plane client plumbing

[bestlux]

Summary

• add a shared control-plane-client crate for default endpoint selection, named-pipe parsing/retry, tonic channel creation, and control-plane client construction
• update CLI and tray to use the shared client helper instead of duplicating Windows named-pipe plumbing
• move named-pipe endpoint tests into the shared crate

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace