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-saas/src/billing/service.rs

@@ -185,8 +185,8 @@ pub async fn increment_usage(
     input_tokens: i64,
     output_tokens: i64,
 ) -> SaasResult<()> {
-    // 确保 quota 行存在（幂等）
-    let _ = get_or_create_usage(pool, account_id).await?;
+    // 确保 quota 行存在（幂等）— 返回值仅用于确认行存在，无需绑定
+    get_or_create_usage(pool, account_id).await?;
 
     // 直接用 account_id + period 原子更新，无需 SELECT 获取 ID
     let now = chrono::Utc::now();