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/orchestrator.py

@@ -104,7 +104,7 @@ class FirecrackerOrchestrator(Orchestrator):
         vm = infra_vm.boot_vm(
             name=ORCHESTRATOR_NAME, slot=netpool.orch_slot(),
             run_dir=infra_vm._orch_dir(), role="orchestrator", mem_mib=_ORCH_MEM_MIB,
-            data_drive=self._ensure_registry_volume(),
+            data_drives=(self._ensure_registry_volume(),),
         )
         # Push the host-canonical signing key (the init waits for it before
         # starting the control plane). It comes through the shared provisioning