feat: filter lost_and_found sentinel from pushes; reset unavail host attrs

- create_lost_and_found(): provide hostattrs_t so makeWorker succeeds
- collect_queues/collect_workers (dataex.c): skip lost_and_found sentinel
  so it is never pushed to apiserver or scheduler
- mark_worker_unavail(): reset hosttype/hostmodel to "unknown" and zero
  all numeric attrs, then update resvals and slots — prevents stale
  capacity from being visible while the host is down
- restore_worker(): apply the same attr reset on WAL recovery so
  restored-but-unavailable workers advertise no capacity until reconnect;
  also remove dead calloc() that was immediately overwritten by makeWorker
- Extract reset_hostattrs_unknown() helper to eliminate copy-paste
- Define XCE_LOST_AND_FOUND_NAME constant in config_repl.h; replace all
  six raw "lost_and_found" string literals across config_repl.c and dataex.c

src/controller/config_repl.h

@@ -13,6 +13,9 @@
 /* Max config blob size — reject before allocating */
 #define CONF_MAX_SIZE (1 * 1024 * 1024)   /* 1 MiB */
 
+/* Sentinel name for the synthetic lost-and-found queue and host */
+#define XCE_LOST_AND_FOUND_NAME "lost_and_found"
+
 /* Compile-time gate for OP_upgradeConfig (Phase 2, default disabled) */
 #ifndef CONF_ENABLE_UPGRADE_CONFIG
 #define CONF_ENABLE_UPGRADE_CONFIG 0