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).
