refactor(pipelock): move PIPELOCK_IMAGE and PIPELOCK_PORT to docker/pipelock.py

Both constants were already only used by Docker-specific code (the
sidecar boot, the proxy_url/host_port naming helpers, the image
contract test). Move them next to DockerPipelockProxy.

Top-level pipelock.py drops the 'os' import along with the constants;
the two test files that pulled PIPELOCK_IMAGE retarget at the new
location.

tests/test_pipelock_image.py

@@ -5,7 +5,7 @@ docker."""
 import subprocess
 import unittest
 
-from claude_bottle.pipelock import PIPELOCK_IMAGE
+from claude_bottle.backend.docker.pipelock import PIPELOCK_IMAGE
 from tests._docker import skip_unless_docker
 
 

tests/test_pipelock_sidecar_smoke.py

@@ -12,8 +12,10 @@ import unittest
 import urllib.request
 from pathlib import Path
 
-from claude_bottle.backend.docker.pipelock import DockerPipelockProxy
-from claude_bottle.pipelock import PIPELOCK_IMAGE
+from claude_bottle.backend.docker.pipelock import (
+    PIPELOCK_IMAGE,
+    DockerPipelockProxy,
+)
 from tests._docker import skip_unless_docker
 from tests.fixtures import fixture_minimal