refactor(docker): split backend.py into prepare / launch / cleanup

Move the resolution, bring-up, and orphan-cleanup logic out of
backend.py into three topic-named modules. DockerBottleBackend becomes
a thin façade that wires the per-instance pipelock proxy and the
provision orchestrator into the free functions.

backend.py drops from ~360 to ~70 lines and each topic now reads
end-to-end in one place. Mirrors the existing provision/ split.

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

claude_bottle/backend/docker/__init__.py

@@ -4,10 +4,14 @@ The bulk of the implementation lives in sibling modules:
 
   - util:                 thin Docker subprocess wrappers
   - network:              Docker network plumbing
+  - pipelock:             DockerPipelockProxy lifecycle
   - bottle_plan:          DockerBottlePlan
   - bottle_cleanup_plan:  DockerBottleCleanupPlan
   - bottle:               DockerBottle handle
-  - backend:              DockerBottleBackend
+  - prepare:              host-side resolution into a DockerBottlePlan
+  - launch:               bring-up + teardown context manager
+  - cleanup:              orphan enumeration, removal, active listing
+  - backend:              DockerBottleBackend façade wiring the above
 
 This file only re-exports the public names so
 `from claude_bottle.backend.docker import DockerBottleBackend` keeps