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- backend/firecracker/consolidated_launch.py: remove superseded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_orchestrator_gateway.py

@@ -427,11 +427,11 @@ class TestDockerGatewayBuild(unittest.TestCase):
         builds = [c for c in calls if c[:2] == ["docker", "build"]]
         self.assertIn("--no-cache", builds[0])
 
-    def test_ensure_built_pulls_when_no_dockerfile(self) -> None:
-        sc = DockerGateway("registry/gateway@sha256:" + "a" * 64, dockerfile=None)
-        with patch(_RUN_DOCKER, return_value=_proc()) as m:
+    def test_ensure_built_noop_when_no_dockerfile(self) -> None:
+        sc = DockerGateway("busybox", dockerfile=None)
+        with patch(_RUN_DOCKER) as m:
             sc.ensure_built()
-        m.assert_called_once_with(["docker", "pull", sc.image_ref])
+        m.assert_not_called()
 
     def test_ensure_built_raises_on_build_failure(self) -> None:
         with patch(_RUN_DOCKER, return_value=_proc(returncode=1, stderr="build boom")):