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/tool/builtin.rs

@@ -7,6 +7,7 @@ mod web_fetch;
 mod execute_skill;
 mod skill_load;
 mod path_validator;
+mod task;
 
 pub use file_read::FileReadTool;
 pub use file_write::FileWriteTool;
@@ -15,6 +16,7 @@ pub use web_fetch::WebFetchTool;
 pub use execute_skill::ExecuteSkillTool;
 pub use skill_load::SkillLoadTool;
 pub use path_validator::{PathValidator, PathValidatorConfig};
+pub use task::TaskTool;
 
 use crate::tool::ToolRegistry;