feat(bottle): per-bottle Dockerfile state + image build hook (PRD 0016)

Phase 1 of PRD 0016. Lays the per-bottle state plumbing that
capability-block remediation will write into:

- claude_bottle/backend/docker/bottle_state.py: bottle_state_dir,
  per_bottle_dockerfile (read), write_per_bottle_dockerfile,
  per_bottle_image_tag (unique per slug), transcript_snapshot_dir.
  Stores under ~/.claude-bottle/state/<slug>/.
- prepare.py: when a per-bottle Dockerfile exists, use
  per_bottle_image_tag(slug) as the base image and pass the
  per-bottle Dockerfile path through DockerBottlePlan.dockerfile_path.
  --cwd still layers a derived image on top.
- launch.py: passes plan.dockerfile_path to build_image so the
  per-bottle Dockerfile is what docker build reads.
- DockerBottlePlan gains dockerfile_path field; print() surfaces it
  in the preflight summary so the operator can see at-a-glance that
  this bottle is running on a rebuilt image.

Phase 2 will write to write_per_bottle_dockerfile (capability-block
approval); Phase 3 wires it into the dashboard.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

claude_bottle/backend/docker/launch.py

@@ -66,7 +66,10 @@ def launch(
             pass
 
     try:
-        docker_mod.build_image(plan.image, _REPO_DIR)
+        docker_mod.build_image(
+            plan.image, _REPO_DIR,
+            dockerfile=plan.dockerfile_path,
+        )
         if plan.derived_image:
             docker_mod.build_image_with_cwd(
                 plan.derived_image, plan.image, plan.spec.user_cwd