refactor(crates): kernel/generation module split + DeerFlow optimizations + middleware + dead code cleanup

- Split zclaw-kernel/kernel.rs (1486 lines) into 9 domain modules
- Split zclaw-kernel/generation.rs (1080 lines) into 3 modules
- Add DeerFlow-inspired middleware: DanglingTool, SubagentLimit, ToolError, ToolOutputGuard
- Add PromptBuilder for structured system prompt assembly
- Add FactStore (zclaw-memory) for persistent fact extraction
- Add task builtin tool for agent task management
- Driver improvements: Anthropic/OpenAI extended thinking, Gemini safety settings
- Replace let _ = with proper log::warn! across SaaS handlers
- Remove unused dependency (url) from zclaw-hands

crates/zclaw-runtime/src/driver/gemini.rs

@@ -265,6 +265,10 @@ impl GeminiDriver {
     /// - Tool definitions use `functionDeclarations`
     /// - Tool results are sent as `functionResponse` parts in `user` messages
     fn build_api_request(&self, request: &CompletionRequest) -> GeminiRequest {
+        if request.thinking_enabled {
+            tracing::debug!("[GeminiDriver] thinking_enabled=true but Gemini does not support native thinking mode; ignoring");
+        }
+
         let mut contents: Vec<GeminiContent> = Vec::new();
 
         for msg in &request.messages {