refactor(pipelock): split sidecar lifecycle into DockerPipelockProxy

PipelockProxy becomes an ABC with the platform-agnostic
prepare/_build_pipelock_yaml as concrete methods and start/stop as
abstract. Docker-specific sidecar lifecycle moves to a new sibling
file:

  claude_bottle/backend/docker/pipelock.py
    DockerPipelockProxy(PipelockProxy) — implements start (docker
    create/cp/network connect/start) and stop (docker inspect/rm -f).

DockerBottleBackend._proxy is now a DockerPipelockProxy instance.
Tests that previously instantiated PipelockProxy() directly switch to
DockerPipelockProxy() (the base is no longer constructable).

tests/test_pipelock_sidecar_smoke.py

@@ -12,7 +12,8 @@ import unittest
 import urllib.request
 from pathlib import Path
 
-from claude_bottle.pipelock import PIPELOCK_IMAGE, PipelockProxy
+from claude_bottle.backend.docker.pipelock import DockerPipelockProxy
+from claude_bottle.pipelock import PIPELOCK_IMAGE
 from tests._docker import skip_unless_docker
 from tests.fixtures import fixture_minimal
 
@@ -38,7 +39,7 @@ class TestPipelockSidecarSmoke(unittest.TestCase):
     )
     def test_smoke(self):
         yaml_path = self.work_dir / "pipelock.yaml"
-        PipelockProxy().prepare(fixture_minimal(), "dev", "demo", yaml_path)
+        DockerPipelockProxy().prepare(fixture_minimal(), "dev", "demo", yaml_path)
 
         create = subprocess.run(
             [