chore: split local side-by-side compose cleanup

[ethanj]

Summary

• split the local side-by-side compose and stale doc cleanup out of the runtime-container work
• keep compare-stack/local-compose changes reviewable on their own
• avoid mixing infra cleanup with the Phase 1A runtime seam refactor

Testing

• not rerun separately after split; this branch contains orthogonal local compose/doc cleanup extracted from the validated Phase 1A stack