feat(llm): re-enable RAM prompt cache (8192 MiB) with --no-cache-idle-slots

--no-cache-idle-slots is the llama.cpp#27148 mitigation for --kv-unified,
which was the cause of misspelled output with --cache-ram on.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.env.example

@@ -5,7 +5,7 @@ LLAMA_MODEL_FILE=Qwen3.8-27B-UD-Q4_K_XL.gguf
 LLAMA_GPU_LAYERS=999
 LLAMA_CTX_SIZE=262144
 LLAMA_PARALLEL=2
-LLAMA_CACHE_RAM=0
+LLAMA_CACHE_RAM=8192
 LLAMA_KV_CACHE_TYPE=q8_0
 
 # Blank = speculative decoding off; see docs/architecture.md#llama-server