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-kernel/src/config.rs

@@ -311,7 +311,7 @@ impl KernelConfig {
     }
 
     /// Find the config file path.
-    fn find_config_path() -> Option<PathBuf> {
+    pub fn find_config_path() -> Option<PathBuf> {
         // 1. Environment variable override
         if let Ok(path) = std::env::var("ZCLAW_CONFIG") {
             return Some(PathBuf::from(path));