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/ai/insight_chat.rs

@@ -70,6 +70,10 @@ pub struct ChatTurnRequest {
     pub top_p: Option<f32>,
     pub top_k: Option<i32>,
     pub min_p: Option<f32>,
+    /// Reasoning toggle for thinking-capable models. Forwarded to the
+    /// llama.cpp backend as `chat_template_kwargs.enable_thinking`; ignored
+    /// by other backends. None defers to the model/template default.
+    pub enable_thinking: Option<bool>,
     pub max_iterations: Option<usize>,
     /// Per-turn system-prompt override. In append mode (default), applied
     /// ephemerally — original system message restored before persistence.
@@ -344,6 +348,7 @@ impl InsightChatService {
             top_p: req.top_p,
             top_k: req.top_k,
             min_p: req.min_p,
+            enable_thinking: req.enable_thinking,
         };
         let backend = self.generator.resolve_backend(kind, &overrides).await?;
         let model_used = backend.model().to_string();
@@ -847,6 +852,7 @@ impl InsightChatService {
             top_p: req.top_p,
             top_k: req.top_k,
             min_p: req.min_p,
+            enable_thinking: req.enable_thinking,
         };
         let backend = self.generator.resolve_backend(kind, &overrides).await?;
         let model_used = backend.model().to_string();
@@ -1017,6 +1023,7 @@ impl InsightChatService {
             top_p: req.top_p,
             top_k: req.top_k,
             min_p: req.min_p,
+            enable_thinking: req.enable_thinking,
         };
         let backend = self.generator.resolve_backend(kind, &overrides).await?;
         let model_used = backend.model().to_string();
@@ -1425,6 +1432,7 @@ impl InsightChatService {
             top_p: req.top_p,
             top_k: req.top_k,
             min_p: req.min_p,
+            enable_thinking: req.enable_thinking,
         };
         let backend = self.generator.resolve_backend(kind, &overrides).await?;
         let model_used = backend.model().to_string();
@@ -1607,6 +1615,7 @@ impl InsightChatService {
             top_p: req.top_p,
             top_k: req.top_k,
             min_p: req.min_p,
+            enable_thinking: req.enable_thinking,
         };
         let backend = self.generator.resolve_backend(kind, &overrides).await?;
         let model_used = backend.model().to_string();