refactor(runtime): loop_runner 双路径合并 — 统一走 middleware chain (Phase 3A)

middleware_chain 从 Option<MiddlewareChain> 改为 MiddlewareChain:
- 移除 6 处 use_middleware 分支 + 2 处 legacy loop_guard inline path
- 移除 loop_guard field + Mutex import + circuit_breaker_triggered 变量
- 空 chain (Default) 行为等价于 middleware path 中的 no-op
- 1154行 → 1023行，净减 131 行
- cargo check --workspace ✓ | cargo test ✓ (排除 desktop 预存编译问题)

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

@@ -83,10 +83,8 @@ impl Kernel {
             loop_runner = loop_runner.with_path_validator(path_validator);
         }
 
-        // Inject middleware chain if available
-        if let Some(chain) = self.create_middleware_chain() {
-            loop_runner = loop_runner.with_middleware_chain(chain);
-        }
+        // Inject middleware chain
+        loop_runner = loop_runner.with_middleware_chain(self.create_middleware_chain());
 
         // Apply chat mode configuration (thinking/reasoning/plan mode)
         if let Some(ref mode) = chat_mode {
@@ -198,10 +196,8 @@ impl Kernel {
             loop_runner = loop_runner.with_path_validator(path_validator);
         }
 
-        // Inject middleware chain if available
-        if let Some(chain) = self.create_middleware_chain() {
-            loop_runner = loop_runner.with_middleware_chain(chain);
-        }
+        // Inject middleware chain
+        loop_runner = loop_runner.with_middleware_chain(self.create_middleware_chain());
 
         // Apply chat mode configuration (thinking/reasoning/plan mode from frontend)
         if let Some(ref mode) = chat_mode {