ComposeStop primitive (currently approximated by single-container Stop)

[bilby91]

Engine.Down on a compose workspace handles Remove: true cleanly (calls ComposeDown → docker compose down). For Remove: false (stop without removing) we approximate by stopping just the primary service's container — sidecars stay running. Documented in down.go:

// We approximate by stopping each container individually since
// our ComposeRuntime interface doesn't expose Stop separately.

Plan

• Add ComposeStop(ctx, ComposeStopSpec) error to runtime.ComposeRuntime
• Implement in runtime/docker/compose.go via docker compose stop — no flags
• Update down.go::downCompose to call ComposeStop when opts.Remove is false
• Integration test: Up project → Down(no remove) → assert ALL services stopped, no containers removed