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

.claude/skills/xce-build/SKILL.md

@@ -0,0 +1,13 @@
+---
+name: xce-build
+description: Build the xce source code
+context: fork
+disable-model-invocation: true
+allowed-tools: Bash(*)
+---
+
+## build xce source code
+
+```bash
+cd src && nix develop --command "rm -Rf xce.build && meson setup xce.build --buildtype=debug -Db_sanitize=address && meson compile -C build"
+```