swarm: HTTP-backed primitive retrieval + live-engram integration test

- primitive_attend retrieves over HTTP (POST /api/search) when ENGRAM_URL is
  set — the location-independent worker model — falling back to the in-process
  store otherwise. Proven against the isolated :8901 clone: CCR compiled a
  bounded context from REAL mind content (VBD/intellectual-dna).
- gate the engram work-tracking mirror behind SWARM_MIRROR=1; the durable
  substrate is always the JSONL journal, so a swarm never depends on the mind
  to track its work. (Repeated POST /api/nodes mirror writes were observed to
  crash the isolated daemon — a daemon-side write-path robustness issue;
  retrieval POST /api/search is solid. Prod :8742 never touched.)
- integ_engram: CCR real-retrieval + full swarm completion against live clone.

lang/swarm/primitives.el

@@ -26,8 +26,20 @@ fn primitive_attend(query: String, limit: Int) -> String {
     if str_eq(query, "") {
         return "[]"
     }
-    // engram_activate returns activated neighbourhood as JSON; scoped by limit.
-    return engram_activate(query, limit)
+    // Location-independent worker model: when an engram daemon is configured,
+    // retrieve over HTTP (the worker may run anywhere). POST /api/search
+    // {query,limit,_auth}. Falls back to the in-process store otherwise.
+    let url: String = env("ENGRAM_URL")
+    if str_eq(url, "") {
+        return engram_activate(query, limit)
+    }
+    let kv: [String] = el_list_empty()
+    let kv = el_list_append(kv, "query")
+    let kv = el_list_append(kv, query)
+    let body0: String = json_build_object(kv)
+    let body1: String = json_set(body0, "limit", int_to_str(limit))
+    let body2: String = json_set_str(body1, "_auth", env("ENGRAM_API_KEY"))
+    return http_post(url + "/api/search", body2)
 }
 
 // ── think — reason over the compiled context ─────────────────────────────────