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/lib.rs

@@ -16,6 +16,7 @@ pub mod stream;
 pub mod growth;
 pub mod compaction;
 pub mod middleware;
+pub mod prompt;
 
 // Re-export main types
 pub use driver::{
@@ -31,3 +32,4 @@ pub use zclaw_growth::VikingAdapter;
 pub use zclaw_growth::EmbeddingClient;
 pub use zclaw_growth::LlmDriverForExtraction;
 pub use compaction::{CompactionConfig, CompactionOutcome};
+pub use prompt::{PromptBuilder, PromptContext, PromptSection};