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_orchestrator.py

@@ -52,7 +52,7 @@ class TestEnsureRunning(unittest.TestCase):
         kw = boot.call_args.kwargs
         self.assertEqual("orchestrator", kw["role"])
         self.assertEqual(4096, kw["mem_mib"])          # orchestrator keeps build headroom
-        self.assertEqual(Path("/reg"), kw["data_drive"])  # DB volume on the CP
+        self.assertEqual((Path("/reg"),), kw["data_drives"])  # DB volume on the CP
         # The host-canonical signing key is pushed to the guest signing-key path.
         self.assertEqual("host-key", push.call_args.args[1])
         self.assertEqual(infra_vm._GUEST_SIGNING_KEY_PATH, push.call_args.args[2])