refactor(pipelock): take stage_dir, derive yaml_path internally

PipelockProxy.prepare now accepts (bottle, slug, stage_dir) and derives
the yaml_path itself, so callers don't need to know the filename.
DockerBottleBackend.prepare_proxy becomes a one-line wrapper whose only
caller already has bottle and slug in scope, so it's inlined and
deleted.

tests/integration/test_pipelock_sidecar_smoke.py

@@ -68,8 +68,7 @@ class TestPipelockSidecarSmoke(unittest.TestCase):
     def test_prepare_and_start_yield_healthy_sidecar(self):
         proxy = DockerPipelockProxy()
 
-        yaml_path = self.work_dir / "pipelock.yaml"
-        prep = proxy.prepare(fixture_minimal().bottles["dev"], self.slug, yaml_path)
+        prep = proxy.prepare(fixture_minimal().bottles["dev"], self.slug, self.work_dir)
 
         self.internal_net = network_create_internal(self.slug)
         self.egress_net = network_create_egress(self.slug)

tests/unit/test_pipelock_yaml.py

@@ -66,11 +66,10 @@ class TestRenderAndWrite(unittest.TestCase):
             self.assertIn(required, text)
 
     def test_prepare_writes_file_at_mode_600(self):
-        yaml_path = self.out_dir / "min.yaml"
-        DockerPipelockProxy().prepare(
-            fixture_minimal().bottles["dev"], "demo", yaml_path
+        plan = DockerPipelockProxy().prepare(
+            fixture_minimal().bottles["dev"], "demo", self.out_dir
         )
-        self.assertEqual(0o600, os.stat(yaml_path).st_mode & 0o777)
+        self.assertEqual(0o600, os.stat(plan.yaml_path).st_mode & 0o777)
 
     def test_prepare_does_not_leak_env_names_or_values(self):
         manifest = Manifest.from_json_obj({
@@ -85,9 +84,10 @@ class TestRenderAndWrite(unittest.TestCase):
             },
             "agents": {"demo": {"skills": [], "prompt": "", "bottle": "dev"}},
         })
-        yaml_path = self.out_dir / "secret.yaml"
-        DockerPipelockProxy().prepare(manifest.bottles["dev"], "demo", yaml_path)
-        content = yaml_path.read_text()
+        plan = DockerPipelockProxy().prepare(
+            manifest.bottles["dev"], "demo", self.out_dir
+        )
+        content = plan.yaml_path.read_text()
         self.assertNotIn("literal-value-should-not-appear", content)
         self.assertNotIn("MY_SECRET", content)
         self.assertNotIn("prompt-message", content)