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/prepare.py

@@ -26,6 +26,11 @@ from .cred_proxy import (
     cred_proxy_url,
 )
 from .git_gate import DockerGitGate, git_gate_container_name
+from .bottle_state import (
+    per_bottle_dockerfile,
+    per_bottle_dockerfile_path,
+    per_bottle_image_tag,
+)
 from .pipelock import DockerPipelockProxy, pipelock_container_name
 from .supervise import DockerSupervise, supervise_container_name
 
@@ -50,7 +55,17 @@ def resolve_plan(
 
     slug = docker_mod.slugify(spec.agent_name)
 
-    image = os.environ.get("CLAUDE_BOTTLE_IMAGE", "claude-bottle:latest")
+    # PRD 0016 capability-block: if a per-bottle Dockerfile has been
+    # written (via apply_capability_change), the base image becomes
+    # per_bottle_image_tag(slug) built from that file. --cwd still
+    # layers a derived image on top.
+    dockerfile_path = ""
+    if per_bottle_dockerfile(slug) is not None:
+        image_default = per_bottle_image_tag(slug)
+        dockerfile_path = str(per_bottle_dockerfile_path(slug))
+    else:
+        image_default = "claude-bottle:latest"
+    image = os.environ.get("CLAUDE_BOTTLE_IMAGE", image_default)
     derived_image = ""
     runtime_image = image
     if spec.copy_cwd:
@@ -184,6 +199,7 @@ def resolve_plan(
         image=image,
         derived_image=derived_image,
         runtime_image=runtime_image,
+        dockerfile_path=dockerfile_path,
         env_file=env_file,
         forwarded_env=forwarded_env,
         prompt_file=prompt_file,