Move all container-runtime-specific logic (binary name, mount args,
stop command, startup check, orphan cleanup) into a single file so
swapping runtimes only requires replacing this one file.
Neutralize "Apple Container" references in comments and docs that
would become incorrect after a runtime swap. References that list
both runtimes as options are left unchanged.
No behavior change — Apple Container remains the default runtime.
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>