feat(v1.8.0): 稳定性修复 + AI Agent 体验增强

稳定性修复:

- 修复 cache.py SQLite 连接泄漏（contextlib.closing 包装）

- 修复 fetch.py requests stream=True 连接泄漏（try/finally resp.close()）

- RETRYABLE_STATUS 新增 403，激活 UA fallback 切换逻辑

- --cache-stats 移至实例解析前，无需实例即可查询

- classify_error 从错误消息提取 HTTP 状态码，正确分类 E_AUTH/E_RATE_LIMIT

- --stream 与 --queries-file 互斥检查，违规报 E_INPUT

- batch 退出码语义统一（0=有结果 / 1=全部错误 / 2=全部空结果）

AI Agent 体验增强:

- 错误码体系完善：E_CONFIG/E_AUTH/E_NETWORK/E_RATE_LIMIT/E_PARSE/E_EMPTY/E_INPUT/E_INTERNAL

- recovery_hint 恢复提示字段，AI Agent 可程序化决策恢复策略

- stream 模式新增 error 事件类型（含 error_code + recovery_hint）

- 进度事件扩展：instance_try/instance_ok/instance_fail

- batch 模式统一 schema（status 字段区分 success/failed）

- JSON 输出含 schema_version 字段确保版本兼容

测试与文档:

- 测试覆盖：330 -> 352

- SKILL.md / README.md 同步更新

scripts/cache.py

@@ -22,6 +22,7 @@ Design notes:
     cache failure must never break a search.
 """
 
+import contextlib
 import hashlib
 import json
 import os
@@ -40,8 +41,15 @@ def _cache_path() -> Path:
     return DEFAULT_CACHE_DIR / "cache.db"
 
 
-def _connect(path: Path) -> sqlite3.Connection:
-    """Open a connection with WAL mode and ensure the schema exists."""
+def _connect(path: Path):
+    """Open a connection with WAL mode and ensure the schema exists.
+
+    Returns a :class:`contextlib.closing` wrapper so ``with _connect(...) as conn:``
+    closes the connection on exit. ``sqlite3.Connection.__exit__`` only commits /
+    rolls back the transaction — it does **not** call ``close()``, which leaks
+    file descriptors across many cache operations (especially under
+    ``--queries-file`` + ``--cache-ttl`` batch runs).
+    """
     path.parent.mkdir(parents=True, exist_ok=True)
     conn = sqlite3.connect(str(path), timeout=10)
     # WAL allows concurrent readers alongside a single writer, which matters
@@ -59,7 +67,7 @@ def _connect(path: Path) -> sqlite3.Connection:
         """
     )
     conn.commit()
-    return conn
+    return contextlib.closing(conn)
 
 
 def _make_key(params: dict) -> str: