feat: 添加 prompt 缓存命中率检测与警告功能

每次 API 请求后自动计算缓存命中率，低于阈值（默认 80%）时在对话流中显示黄色警告消息。
同时更新 /context 命令输出中显示缓存命中率。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/ContextVisualization.tsx

@@ -115,6 +115,8 @@ export function ContextVisualization({ data }: Props): React.ReactNode {
     agents,
     skills,
     messageBreakdown,
+    cacheHitRate,
+    cacheThreshold,
   } = data;
 
   // Filter out categories with 0 tokens for the legend, and exclude Free space, Autocompact buffer, and deferred
@@ -166,6 +168,12 @@ export function ContextVisualization({ data }: Props): React.ReactNode {
             {model} · {formatTokens(totalTokens)}/{formatTokens(rawMaxTokens)} tokens ({percentage}%)
           </Text>
           <CollapseStatus />
+          {cacheHitRate !== undefined && cacheThreshold !== undefined && (
+            <Text color={cacheHitRate < cacheThreshold ? 'warning' : undefined}>
+              Cache hit rate: {cacheHitRate.toFixed(0)}%
+              {cacheHitRate < cacheThreshold ? ` (below ${cacheThreshold}% threshold)` : ''}
+            </Text>
+          )}
           <Text> </Text>
           <Text dimColor italic>
             Estimated usage by category