feat: add startup check with enabled/startupCheckEnabled/failFastOnUnavailable config

- OpenCodeHttpClientConfig: add enabled, startupCheckEnabled, failFastOnUnavailable
- OpenCodeCliConfig: add enabled, startupCheckEnabled, failFastOnUnavailable
- OpenCodeClient: refactor constructors (8 overloads + full DI)
  - enabled=false short-circuits subsystem initialization (fields null)
  - Startup checks run during main constructor (HTTP health + CLI probe)
  - isHttpEnabled() / isCliEnabled() for post-construction queries
- Add OpenCodeClientConstructionTest (7 tests)

Author: loong10k

src/main/java/io/github/easy4j/opencode/OpenCodeCliConfig.java

@@ -11,6 +11,23 @@
 @Data
 public class OpenCodeCliConfig {
 
+    /**
+     * 是否启用本地 CLI 子系统。
+     * <p>为 false 时跳过 CLI 相关初始化和检查。</p>
+     */
+    private boolean enabled = true;
+
+    /**
+     * 启动时是否探测 {@code opencode --version}。
+     */
+    private boolean startupCheckEnabled = true;
+
+    /**
+     * CLI 不可用时是否快速失败（中断构造）。
+     * <p>默认 false 仅打 WARN；生产环境建议设为 true。</p>
+     */
+    private boolean failFastOnUnavailable = false;
+
     /**
      * 本地可执行文件名或绝对路径。
      */