feat(git-gate): wire DockerGitGate through prepare/launch/plan

DockerBottleBackend now instantiates a DockerGitGate alongside
DockerPipelockProxy and DockerSSHGate; the prepare step lifts
bottle.git into a GitGatePlan stored on DockerBottlePlan, and
launch starts/stops the sidecar in the same ExitStack as the
other two (only when bottle.git is non-empty).

bottle_plan.print now surfaces git remotes and per-upstream gate
forwards in the y/N preflight; to_dict adds git_remotes and
git_gate keys to the dry-run JSON payload for CLI consumers.
PRD: docs/prds/0008-git-gate.md

tests/integration/test_dry_run_plan.py

@@ -81,6 +81,8 @@ class TestDryRunPlan(unittest.TestCase):
             self.assertEqual([], plan["skills"])
             self.assertEqual([], plan["ssh_hosts"])
             self.assertEqual([], plan["ssh_gate"])
+            self.assertEqual([], plan["git_remotes"])
+            self.assertEqual([], plan["git_gate"])
             self.assertEqual(False, plan["remote_control"])
             self.assertEqual(0, plan["prompt"]["length"])