feat(middleware): add butler router for semantic skill routing

New ButlerRouterMiddleware (priority 80) intercepts user messages,
classifies intent using keyword-based domain detection, and injects
routing context into the system prompt. Supports healthcare, data
report, policy compliance, and meeting coordination domains.

- New: butler_router.rs — keyword classifier + MiddlewareContext injection
- Registered in Kernel::create_middleware_chain() at priority 80
- 9 tests passing (classification + middleware integration)

crates/zclaw-kernel/src/kernel/mod.rs

@@ -190,6 +190,13 @@ impl Kernel {
     pub(crate) fn create_middleware_chain(&self) -> Option<zclaw_runtime::middleware::MiddlewareChain> {
         let mut chain = zclaw_runtime::middleware::MiddlewareChain::new();
 
+        // Butler router — semantic skill routing context injection
+        {
+            use std::sync::Arc;
+            let mw = zclaw_runtime::middleware::butler_router::ButlerRouterMiddleware::new();
+            chain.register(Arc::new(mw));
+        }
+
         // Data masking middleware — mask sensitive entities before any other processing
         {
             use std::sync::Arc;