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/common.py

@@ -60,8 +60,12 @@ def setup_logging(verbose: bool = False, quiet: bool = False) -> None:
 MAX_RETRIES = 3
 RETRY_BACKOFF_BASE = 1.5  # seconds; exponential backoff + jitter
 
-# HTTP status codes that are worth retrying (rate limit + gateway errors)
-RETRYABLE_STATUS = frozenset({429, 502, 503, 504})
+# HTTP status codes worth retrying. 403 is included so fetch_url's UA-fallback
+# loop can kick in when a site blocks the default searxng-cli User-Agent
+# (a fresh UA is tried on each retry attempt). search.py also retries 403 —
+# it doesn't switch UAs, so true auth failures waste ~3 attempts, accepted
+# as a trade-off for one shared retry policy across both scripts.
+RETRYABLE_STATUS = frozenset({403, 429, 502, 503, 504})
 
 # Browser-like UA strings for fallback when the searxng-cli UA is blocked
 FALLBACK_UAS = [
@@ -302,6 +306,29 @@ E_EMPTY = "E_EMPTY"
 E_INPUT = "E_INPUT"
 E_INTERNAL = "E_INTERNAL"
 
+# 每个 E_* 配套的可操作恢复建议，让 AI Agent 能自决策下一步动作，
+# 而不是盲目重试或放弃。在 _emit_error 的 JSON 输出中作为 recovery_hint 字段。
+RECOVERY_HINTS = {
+    E_CONFIG: "Provide -i/--instance, set SEARXNG_INSTANCE env var, or create "
+              "searxng.toml/instances.txt config file.",
+    E_AUTH: "Verify --auth-bearer/--auth-basic credentials or "
+            "SEARXNG_BEARER_TOKEN/SEARXNG_BASIC_AUTH env vars. Check token "
+            "expiry and instance access permissions.",
+    E_NETWORK: "Retry with backoff, or try a different SearXNG instance. "
+               "Check network connectivity, proxy settings, and instance uptime.",
+    E_RATE_LIMIT: "Wait before retrying (exponential backoff). Reduce query "
+                  "frequency, narrow --time-range, or distribute load across "
+                  "multiple instances.",
+    E_INPUT: "Check query syntax, --categories values, --time-range format, "
+             "and flag combinations. Use --help for valid options.",
+    E_PARSE: "The instance returned malformed data. Try a different instance, "
+             "switch --method, or check if the instance version is compatible.",
+    E_EMPTY: "Refine the query (more specific terms), broaden --time-range, "
+             "add --categories, or increase --pageno to find more results.",
+    E_INTERNAL: "This is likely a bug. Re-run with --verbose and report the "
+                "full output for diagnosis.",
+}
+
 
 def classify_error(exc: BaseException) -> str:
     """将异常分类为结构化错误码，供 AI Agent 程序化处理。
@@ -350,18 +377,36 @@ def classify_error(exc: BaseException) -> str:
     if isinstance(exc, (ValueError, _json.JSONDecodeError)):
         return E_PARSE
 
-    # RuntimeError：从消息中推断（search_multi 的 "All N instances failed" 等）
+    # RuntimeError：从消息中推断（search_multi 的 "All N instances failed.
+    # Last error: HTTP Error 403: Forbidden" 等）。并行路径现在会把 last_error
+    # 拼进消息，让这里能提取真实错误类型，而不是一律误判 E_NETWORK。
     msg = str(exc).lower()
     if isinstance(exc, RuntimeError):
+        # 先检查 auth/rate-limit 关键字（最常见，来自 last_error 详情）
         if "auth" in msg or "403" in msg or "401" in msg:
             return E_AUTH
         if "rate" in msg or "429" in msg:
             return E_RATE_LIMIT
+        # 从 "http error NNN" / "http NNN" 模式中提取状态码，
+        # 正确分类 "All instances failed. Last error: HTTP Error 403" 等
+        import re as _re
+        status_match = _re.search(r'http(?: error)? (\d{3})', msg)
+        if status_match:
+            status = int(status_match.group(1))
+            if status == 429:
+                return E_RATE_LIMIT
+            if status in (401, 403):
+                return E_AUTH
+            if 400 <= status < 500:
+                return E_INPUT
+            if 500 <= status < 600:
+                return E_NETWORK
+        # 所有实例失败的通用模式（无更具体的 HTTP 状态码时才判为网络错误）
         if "all" in msg and "instance" in msg and "fail" in msg:
             return E_NETWORK
         if "parse" in msg or "json" in msg or "html" in msg:
             return E_PARSE
-        if "not found" in msg or "no " in msg and "instance" in msg:
+        if "not found" in msg or ("no " in msg and "instance" in msg):
             return E_CONFIG
         return E_INTERNAL