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-runtime/src/middleware.rs

@@ -265,6 +265,7 @@ impl Default for MiddlewareChain {
 // Sub-modules — concrete middleware implementations
 // ---------------------------------------------------------------------------
 
+pub mod butler_router;
 pub mod compaction;
 pub mod dangling_tool;
 pub mod data_masking;