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/bin/populate_knowledge.rs

@@ -336,6 +336,7 @@ async fn main() -> anyhow::Result<()> {
                 args.top_p,
                 args.top_k,
                 args.min_p,
+                None, // enable_thinking: leave model/template default
                 args.max_iterations,
                 None,
                 Vec::new(),