feat(controller): OPCode_probeConfigTag handler with Request envelope reply (bug-1506)

- Add OPCode_probeConfigTag dispatch in cluster.c
- Implement probe_config_tag handler returning config tag via
  standard xce_reply() → RequestReply envelope
- Populate redirectHost/redirectPort for follow_redirects support
- Wire probeConfigTag reply through typemap
- Add probeConfigTagReply struct to controller.capnp schema

src/controller/cluster.c

@@ -816,6 +816,15 @@ static int myreq_handler(vtask_t* task)
         }
         break;
 #endif
+	case OPCode_probeConfigTag:
+		/* bug-1506: return the current config generation tag for xceadmin's
+		 * config push --dry-run.  Read-only — no WAL commit, no SHA
+		 * verification.  Any role (PRIMARY or STANDBY) can service this;
+		 * ha_dispatch_gate already allows OPCode_probeConfigTag on every
+		 * non-INIT role.  Redirect hints are filled inside the handler. */
+		rc = handle_probe_config_tag(c, ep, &request);
+		break;
+
     case OPCode_schedAdmin: {
         /* Stateless pass-through forward to the scheduler (task 10.1, Req
          * 5.10). The controller commits no WAL entry and interprets no