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- backend/firecracker/consolidated_launch.py: remove superseded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

@@ -151,6 +151,31 @@ class TestReprovisionGateway(unittest.TestCase):
                 self.c.reprovision_gateway("b1", "key")
 
 
+class TestUpdateAgentSecret(unittest.TestCase):
+    def setUp(self) -> None:
+        self.c = OrchestratorClient("http://orch:8080")
+
+    def test_success_posts_single_secret(self) -> None:
+        with patch(_URLOPEN, return_value=_resp(200, {"updated": True})) as opened:
+            self.assertTrue(self.c.update_agent_secret("b1", "A", "fresh", "key"))
+        request = opened.call_args.args[0]
+        self.assertEqual("POST", request.get_method())
+        self.assertTrue(request.full_url.endswith("/bottles/b1/secret"))
+        self.assertEqual(
+            {"name": "A", "value": "fresh", "env_var_secret": "key"},
+            json.loads(request.data),
+        )
+
+    def test_unknown_bottle_is_false(self) -> None:
+        with patch(_URLOPEN, side_effect=_http_error(404)):
+            self.assertFalse(self.c.update_agent_secret("b1", "A", "v", "key"))
+
+    def test_other_status_raises(self) -> None:
+        with patch(_URLOPEN, side_effect=_http_error(400)):
+            with self.assertRaises(OrchestratorClientError):
+                self.c.update_agent_secret("b1", "A", "v", "key")
+
+
 class TestHealthAndPolicy(unittest.TestCase):
     def setUp(self) -> None:
         self.c = OrchestratorClient("http://orch:8080")