AI: add enable_thinking reasoning toggle plumbed to llama.cpp

New optional SamplingOverride forwarded to llama-server as
chat_template_kwargs.enable_thinking (gates Qwen3-style reasoning
blocks). None leaves the template default; other backends ignore it.
Wired through the agentic-insight and chat-turn request bodies/handlers.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/unified_search.rs

@@ -193,6 +193,7 @@ pub async fn unified_search<TagD: TagDao>(
         top_p: None,
         top_k: None,
         min_p: None,
+        enable_thinking: None,
     };
     let backend = match state
         .insight_generator