fix(gateway): persist git-gate state across gateway restarts

Per-bottle git-gate state (bare repos under /git/<id>, deploy creds under
/git-gate/creds/<id>) was provisioned once at bottle launch and lived only
in the gateway's ephemeral storage. A gateway rebuild/restart wiped it and
nothing re-provisioned already-running bottles, so their agents 404'd on
fetch/push. Same class of bug as the CA (#510); the orchestrator restores
only egress tokens, not git-gate declarations.

Persist the state on both backends, mirroring the CA-persistence approach:

- firecracker: attach a second persistent data drive (/dev/vdc) to the
  gateway VM and bind-mount its git/ + creds/ subdirs onto /git and
  /git-gate/creds in the gateway guest init, before the data plane starts.
  Generalize the VM config to a stable-ordered data_drives tuple (CA=vdb,
  git=vdc; orchestrator registry stays vdb).
- docker: bind-mount host dirs (host_gateway_git_dir / creds_dir, under the
  never-pruned app-data root) onto /git and /git-gate/creds, with
  BOT_BOTTLE_DOCKER_GIT_MOUNT / _CREDS_MOUNT env overrides so CI isolates
  them to per-run volumes it cleans up.

Teardown already rm -rf's /git/<id> + creds, so the persistent store
self-cleans over the normal lifecycle.

Closes #512

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/unit/test_firecracker_backend.py

@@ -404,16 +404,19 @@ class TestBootArgs(unittest.TestCase):
         self.assertEqual("bbfc0", cfg["network-interfaces"][0]["host_dev_name"])
         self.assertEqual(1, len(cfg["drives"]))  # no data drive by default
 
-    def test_config_adds_data_drive(self):
+    def test_config_adds_data_drives_in_order(self):
         cfg = cast(Any, firecracker_vm._config(
             rootfs=Path("/run/rootfs.ext4"), tap="bbfc0",
             guest_ip="100.64.0.1", host_ip="100.64.0.0", pubkey="k",
             vcpus=2, mem_mib=2048, guest_mac="06:00:AC:10:00:02",
-            data_drive=Path("/run/registry.ext4"),
+            data_drives=(Path("/run/ca.ext4"), Path("/run/git.ext4")),
         ))
-        self.assertEqual(2, len(cfg["drives"]))
+        # rootfs (vda) + two data drives, attached in list order so the guest
+        # sees them as /dev/vdb, /dev/vdc — an order callers depend on.
+        self.assertEqual(3, len(cfg["drives"]))
         self.assertFalse(cfg["drives"][1]["is_root_device"])
-        self.assertEqual("/run/registry.ext4", cfg["drives"][1]["path_on_host"])
+        self.assertEqual("/run/ca.ext4", cfg["drives"][1]["path_on_host"])
+        self.assertEqual("/run/git.ext4", cfg["drives"][2]["path_on_host"])
 
 
 class TestBottleExecClose(unittest.TestCase):