feat(prd-0081): reprovision CA, git-gate, and egress tokens on gateway bring-up

On a Firecracker gateway cold boot, reconcile every live agent VM against
the fresh gateway: push the new mitmproxy CA into each agent's trust store,
re-provision git-gate repos/creds from the persisted upstreams snapshot,
and restore egress tokens. Per-bottle failures are logged and skipped so
one unreachable VM does not block the rest.

New modules / changes:
- backend/firecracker/reconcile.py: attach_bottled_agents_to_gateway,
  _push_ca, _reprovision_git_gate, _guest_ip_from_config
- git_gate/provision.py: write upstreams.json after key provisioning so
  the bring-up reconcile can reconstruct the upstream table without the
  manifest
- backend/firecracker/infra.py: call attach_bottled_agents_to_gateway in
  the cold-boot branch of ensure_running()
- backend/base.py: no-op default on BottleBackend
- backend/firecracker/consolidated_launch.py: remove superseded
  _reprovision_running_bottles / _guest_ip_from_config
- orchestrator: OrchestratorCore.update_agent_secret + POST
  /bottles/<id>/secret + client.update_agent_secret (single-secret
  in-place update, the reusable primitive from the 2026-07-26 hotfix)
- tests: 15 new tests in test_firecracker_reconcile.py; updated
  test_firecracker_infra.py cold-boot cases; stale FC reprovision tests
  removed from test_backend_secret_reprovision.py

Closes #516.

tests/unit/test_docker_orchestrator.py

@@ -231,12 +231,11 @@ class TestDockerOrchestrator(unittest.TestCase):
             with self.assertRaises(GatewayError):
                 self.orch.ensure_built()
 
-    def test_ensure_built_pulls_without_dockerfile(self) -> None:
-        orch = DockerOrchestrator("registry/orchestrator@sha256:" + "a" * 64,
-                                  dockerfile=None)
-        with patch(_RUN, return_value=_proc()) as run:
+    def test_ensure_built_noop_without_dockerfile(self) -> None:
+        orch = DockerOrchestrator(dockerfile=None)
+        with patch(_RUN) as run:
             orch.ensure_built()
-        run.assert_called_once_with(["docker", "pull", orch.image_ref])
+        run.assert_not_called()
 
     def test_is_running_reads_docker_ps(self) -> None:
         with patch(_RUN, return_value=_proc(stdout=ORCHESTRATOR_NAME)):