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/utils/settings/types.ts

@@ -1072,6 +1072,15 @@ export const SettingsSchema = lazySchema(() =>
             'Only applies to User, Project, and Local memory types (Managed/policy files cannot be excluded). ' +
             'Examples: "/home/user/monorepo/CLAUDE.md", "**/code/CLAUDE.md", "**/some-dir/.claude/rules/**"',
         ),
+      cacheThreshold: z
+        .number()
+        .int()
+        .min(0)
+        .max(100)
+        .optional()
+        .describe(
+          'Prompt cache hit rate threshold (0-100). Warnings shown when cache hit rate falls below this percentage. Default: 80.',
+        ),
       pluginTrustMessage: z
         .string()
         .optional()