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- backend/firecracker/consolidated_launch.py: remove superseded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

@@ -125,6 +125,47 @@ class TestDispatch(unittest.TestCase):
             {"EGRESS_TOKEN_0": "upstream-secret"}, self.orch.tokens_for(bottle_id),
         )
 
+    def test_update_agent_secret_in_place(self) -> None:
+        key = base64.urlsafe_b64encode(b"unit-test-key").rstrip(b"=").decode()
+        status, payload = dispatch(
+            self.orch, "POST", "/bottles", _body({
+                "source_ip": "10.243.0.21",
+                "tokens": {"A": "old", "B": "keep"},
+                "env_var_secret": key,
+            }),
+        )
+        self.assertEqual(201, status)
+        bottle_id = payload["bottle_id"]
+        assert isinstance(bottle_id, str)
+        status, response = dispatch(
+            self.orch, "POST", f"/bottles/{bottle_id}/secret",
+            _body({"name": "A", "value": "fresh", "env_var_secret": key}),
+        )
+        self.assertEqual((200, {"updated": True}), (status, response))
+        self.assertEqual({"A": "fresh", "B": "keep"}, self.orch.tokens_for(bottle_id))
+
+    def test_update_agent_secret_validates_and_unknown_bottle(self) -> None:
+        status, _ = dispatch(self.orch, "POST", "/bottles/b1/secret", b"not-json")
+        self.assertEqual(400, status)
+        status, _ = dispatch(
+            self.orch, "POST", "/bottles/b1/secret", _body({"name": "A"}),
+        )
+        self.assertEqual(400, status)  # value + env_var_secret missing
+        status, _ = dispatch(
+            self.orch, "POST", "/bottles/ghost/secret",
+            _body({"name": "A", "value": "v", "env_var_secret": "k"}),
+        )
+        self.assertEqual(404, status)
+
+    def test_update_agent_secret_is_cli_only(self) -> None:
+        # A data-plane (`gateway`) caller must not set a bottle's tokens.
+        status, _ = dispatch(
+            self.orch, "POST", "/bottles/b1/secret",
+            _body({"name": "A", "value": "v", "env_var_secret": "k"}),
+            role=ROLE_GATEWAY,
+        )
+        self.assertEqual(403, status)
+
     def test_reprovision_validates_request_and_missing_rows(self) -> None:
         status, _ = dispatch(
             self.orch, "POST", "/bottles/b1/reprovision_gateway", b"not-json",