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/include/typemap.h

@@ -197,6 +197,14 @@ typedef struct {
     uint64_t peer_last_log_lsn;
 } request_vote_reply_t;
 
+typedef struct {
+    char *name;
+} probe_config_tag_req_t;
+
+typedef struct {
+    uint64_t config_tag;
+} probe_config_tag_reply_t;
+
 /* Heartbeat reply state report — carries standby's applied_lsn and config_tag,
  * plus its raft term so the primary can discover a higher term it lost track
  * of (bug-1421 term-discovery: an AE reject response never reaches the raft
@@ -291,6 +299,7 @@ typedef struct {
         xce_ctrlstatus_t*          ctrlstatus;
         xce_fetchfile_reply_t*     fetchfilereply;
         sched_admin_reply_t*       schedadminreplymsg;
+        probe_config_tag_reply_t*  probeconfigtagreplymsg;
     } data;
     /* Req 7: standby redirect fields — populated when status == XCE_ERR_STANDBY */
     char    *redirect_host;
@@ -331,6 +340,7 @@ typedef struct {
         snapshot_transfer_t*      snapshottransfermsg;
         xce_fetchfile_t*          fetchfilemsg;
         sched_admin_req_t*        schedadminmsg;
+        probe_config_tag_req_t*    probeconfigtagrequestmsg;
     } body;
 } xce_request_t;
 
@@ -341,14 +351,6 @@ typedef struct {
  * request_vote_req_t and request_vote_reply_t are defined above
  * xce_reply_t so they can be used in the xce_request_t body union. */
 
-typedef struct {
-    char *name;
-} probe_config_tag_req_t;
-
-typedef struct {
-    uint64_t config_tag;
-} probe_config_tag_reply_t;
-
 typedef struct {
     uint64_t term;
     uint64_t lsn;