feat(worker-join-refactor): implement apply_join_state, init_worker, rejoin_worker

Replace monolithic add_worker with three focused functions:
- apply_join_state (static under production, exported under XCE_TESTING):
  shared status/resource update called by both create and rejoin paths;
  clears XCE_WORKER_UNAVAIL, applies hconfig overrides, updates resources
- init_worker: first-time join path via makeWorker + apply_join_state
- rejoin_worker: reconnect path with explicit admin-bit preservation
  (XCE_WORKER_ADMIN_MASK = LOCKEDU|CLOSED|HIDDEN|CLOSED_BY_ADMIN)

Also makes find_matched_hostconfig non-static so join.c can call it
via the extern declaration already in worker.h.

Removes add_worker (no longer declared in worker.h, no callers).

All 7 tests in test_worker_join pass:
  apply_join_state (clears UNAVAIL, hosttype override, resource values),
  init_worker (creates entry, returns correct idx),
  rejoin_worker (updates attrs/ep, preserves admin bits).

.kiro/specs/worker-join-refactor/spec.json

@@ -0,0 +1,22 @@
+{
+  "feature_name": "worker-join-refactor",
+  "created_at": "2026-05-15T00:00:00Z",
+  "updated_at": "2026-05-15T03:20:00Z",
+  "language": "en",
+  "phase": "tasks-generated",
+  "approvals": {
+    "requirements": {
+      "generated": true,
+      "approved": true
+    },
+    "design": {
+      "generated": true,
+      "approved": true
+    },
+    "tasks": {
+      "generated": true,
+      "approved": true
+    }
+  },
+  "ready_for_implementation": true
+}