Make the router's HF-cache scan opt-in.

By default `llama-server` in router mode scans the HF cache and auto-registers
every downloaded model under a forced `repo:tag` id, producing untuned
duplicates of models already defined via --models-preset and preventing free
naming. The scan is purely a listing convenience and is independent of
auto-download on first use (that goes through the child process's --hf-repo).

Gate it behind LLAMA_ROUTER_SCAN_CACHE so presets are the single source of
truth by default; set the env var to restore upstream behaviour.

--- a/tools/server/server-models.cpp
+++ b/tools/server/server-models.cpp
@@ -280,8 +280,13 @@
 
 void server_models::load_models() {
     // Phase 1: load presets from all sources — pure I/O, no lock needed
-    // 1. cached models
-    common_presets cached_models = ctx_preset.load_from_cache();
+    // 1. cached models (opt-in). Scanning the HF cache auto-registers untuned
+    //    twins of models already defined as presets and forces their id to
+    //    repo:tag. Off by default; set LLAMA_ROUTER_SCAN_CACHE to restore it.
+    common_presets cached_models;
+    if (std::getenv("LLAMA_ROUTER_SCAN_CACHE")) {
+        cached_models = ctx_preset.load_from_cache();
+    }
     SRV_INF("Loaded %zu cached model presets\n", cached_models.size());
     // 2. local models from --models-dir
     common_presets local_models;
