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>

bot_bottle/backend/firecracker/firecracker_vm.py

@@ -84,7 +84,7 @@ def _config(
     vcpus: int,
     mem_mib: int,
     guest_mac: str,
-    data_drive: Path | None = None,
+    data_drives: tuple[Path, ...] = (),
     extra_boot_args: str = "",
 ) -> dict[str, object]:
     drives: list[dict[str, object]] = [
@@ -95,12 +95,15 @@ def _config(
             "is_read_only": False,
         }
     ]
-    # A second virtio-block device (guest /dev/vdb) — the infra VM's
-    # persistent registry "volume", a host-side ext4 file that outlives the
-    # ephemeral rootfs across VM restarts.
-    if data_drive is not None:
+    # Extra virtio-block devices — the infra VMs' persistent "volumes",
+    # host-side ext4 files that outlive the ephemeral rootfs across VM restarts.
+    # They appear to the guest as /dev/vdb, /dev/vdc, ... in list order (after
+    # the root /dev/vda), so callers must keep the order stable: the orchestrator
+    # attaches its registry (vdb); the gateway attaches its CA (vdb) then its
+    # git-gate state (vdc).
+    for i, data_drive in enumerate(data_drives):
         drives.append({
-            "drive_id": "data",
+            "drive_id": f"data{i}",
             "path_on_host": str(data_drive),
             "is_root_device": False,
             "is_read_only": False,
@@ -138,7 +141,7 @@ def boot(
     mem_mib: int = 2048,
     guest_mac: str = "06:00:AC:10:00:02",
     detached: bool = False,
-    data_drive: Path | None = None,
+    data_drives: tuple[Path, ...] = (),
     extra_boot_args: str = "",
 ) -> VmHandle:
     """Write the config and launch the VMM. Returns once the process is
@@ -155,7 +158,7 @@ def boot(
         _config(
             rootfs=rootfs, tap=tap, guest_ip=guest_ip, host_ip=host_ip,
             pubkey=pubkey, vcpus=vcpus, mem_mib=mem_mib, guest_mac=guest_mac,
-            data_drive=data_drive, extra_boot_args=extra_boot_args,
+            data_drives=data_drives, extra_boot_args=extra_boot_args,
         ),
         indent=2,
     ))