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 {

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

@@ -201,7 +201,7 @@ impl Kernel {
     /// When middleware is configured, cross-cutting concerns (compaction, loop guard,
     /// token calibration, etc.) are delegated to the chain. When no middleware is
     /// registered, the legacy inline path in `AgentLoop` is used instead.
-    pub(crate) fn create_middleware_chain(&self) -> Option<zclaw_runtime::middleware::MiddlewareChain> {
+    pub(crate) fn create_middleware_chain(&self) -> zclaw_runtime::middleware::MiddlewareChain {
         let mut chain = zclaw_runtime::middleware::MiddlewareChain::new();
 
         // Butler router — semantic skill routing context injection
@@ -359,13 +359,11 @@ impl Kernel {
             chain.register(Arc::new(mw));
         }
 
-        // Only return Some if we actually registered middleware
-        if chain.is_empty() {
-            None
-        } else {
+        // Always return the chain (empty chain is a no-op)
+        if !chain.is_empty() {
             tracing::info!("[Kernel] Middleware chain created with {} middlewares", chain.len());
-            Some(chain)
         }
+        chain
     }
 
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