refactor(sidecars): drop vestigial start/stop methods (PRD 0024 chunk 3)

Compose-up has owned per-container lifecycle since PRD 0018 ch3;
the .start() / .stop() methods on DockerPipelockProxy /
DockerEgress / DockerGitGate / DockerSupervise (and their
abstractmethod declarations in the four base ABCs) were already
documented as vestigial. With the bundle path in flight
(PRD 0024 ch2), they are truly dead — collapse to nothing.

Changes:
- Removed start/stop methods from the four DockerSidecar
  classes. Plan dataclasses, image/path constants,
  container-name helpers, and the .prepare() methods all stay
  (the renderer + apply path still need them).
- Removed the matching @abstractmethod declarations in the
  base ABCs so concrete subclasses don't have to stub them.
- launch.launch() and prepare.resolve_plan() no longer take
  proxy/git_gate/egress/supervise instance parameters. backend.py
  loses the four instance attributes it threaded through.
  prepare.resolve_plan() instantiates the four classes itself
  to call their .prepare() methods.
- Deleted four integration tests that only exercised the
  removed lifecycle: test_pipelock_sidecar_smoke,
  test_supervise_sidecar, test_git_gate_sidecar,
  test_git_gate_mirror.
- Dropped the .stop-idempotency case in test_orphan_cleanup;
  the network-cleanup cases stay (those test real production
  code).
- Marked test_pipelock_apply @skip pending chunk 4 — its
  bringup helper used .start; chunk 4 rewrites it with direct
  `docker run`.

Dockerfile deletion deferred to chunk 5 (when the bundle flag
default flips) — the legacy compose path still needs
Dockerfile.{egress,git-gate,supervise} until then.

Net: 708 lines removed, 80 added.

533 unit tests + 27 integration tests passing (5 skipped: the
chunk-4-pending case + existing GITEA_ACTIONS guards).

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

tests/integration/test_pipelock_apply.py

@@ -50,6 +50,11 @@ from tests.fixtures import fixture_minimal
     "skipped under act_runner: pipelock_tls_init uses a host bind mount "
     "that doesn't share fs with the runner container",
 )
+@unittest.skip(
+    "PRD 0024 chunk 3: the .start-based bringup helper this test used was "
+    "deleted. Chunk 4 rewrites the bringup with a direct `docker run` so "
+    "the apply_allowlist_change hot-reload retains integration coverage."
+)
 class TestPipelockApply(unittest.TestCase):
     def setUp(self):
         self.slug = f"cb-test-pla-{os.getpid()}-{int(time.time())}"