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- backend/firecracker/consolidated_launch.py: remove superseded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

@@ -103,6 +103,25 @@ class TestOrchestrator(unittest.TestCase):
         self.assertTrue(self.orch.reprovision_from_secret(rec.bottle_id, key))
         self.assertEqual({"EGRESS_TOKEN_0": "secret"}, self.orch.tokens_for(rec.bottle_id))
 
+    def test_update_agent_secret_refreshes_one_token_in_place(self) -> None:
+        key = new_env_var_secret()
+        rec = self.orch.launch_bottle(
+            "10.243.0.20", tokens={"A": "old-a", "B": "keep-b"}, env_var_secret=key,
+        )
+        self.assertTrue(self.orch.update_agent_secret(rec.bottle_id, "A", "new-a", key))
+        # In memory: A refreshed, B left untouched.
+        self.assertEqual({"A": "new-a", "B": "keep-b"}, self.orch.tokens_for(rec.bottle_id))
+        # At rest: the whole set stays decryptable with the SAME env_var_secret,
+        # so a later reprovision restores the refreshed value (not the launch one).
+        self.orch._tokens.clear()
+        self.assertTrue(self.orch.reprovision_from_secret(rec.bottle_id, key))
+        self.assertEqual({"A": "new-a", "B": "keep-b"}, self.orch.tokens_for(rec.bottle_id))
+
+    def test_update_agent_secret_unknown_bottle_is_false(self) -> None:
+        self.assertFalse(
+            self.orch.update_agent_secret("ghost", "A", "v", new_env_var_secret())
+        )
+
     def test_reprovision_rejects_missing_rows_and_wrong_key(self) -> None:
         self.assertFalse(self.orch.reprovision_from_secret("missing", new_env_var_secret()))
         key = "AQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQE"