PRD 0018 chunk 1. New module `claude_bottle/backend/docker/compose.py`
exposing `bottle_plan_to_compose(plan) -> dict` — a pure function that
translates a fully-resolved DockerBottlePlan into a Compose v2 spec.
Not wired in yet. Tests cover the conditional-service matrix (git
on/off × egress on/off × supervise on/off) plus per-service shape
(images vs builds, network aliases, bind mounts, env vars, depends_on).
Draft a PRD that replaces the chain of per-sidecar docker SDK calls
in `claude-bottle start` with a single `docker compose` project per
instance. Each `state/<slug>/` dir gets a self-describing set of
artifacts: metadata.json, docker-compose.yml, compose.log, and the
existing transcript/ + live-config/.