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-protocols/src/mcp_types.rs

@@ -278,7 +278,8 @@ pub struct PromptMessage {
 
 // === Content Blocks ===
 
-/// Content block for tool results and messages
+/// MCP protocol wire format content block. Used for Model Context Protocol resource responses.
+/// Distinct from zclaw_types::ContentBlock (LLM messages) and zclaw_hands::ContentBlock (presentations).
 #[derive(Debug, Clone, Deserialize)]
 #[serde(tag = "type", rename_all = "snake_case")]
 pub enum ContentBlock {