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/fetch.py

@@ -513,35 +513,42 @@ def fetch_url(url: str, timeout: int = 15, user_agent: str = None,
             if _HAS_REQUESTS:
                 resp = _requests.get(url, timeout=timeout, headers=headers,
                                      allow_redirects=allow_redirects, stream=True)
-                resp.raise_for_status()
+                # stream=True holds the socket open; must close explicitly,
+                # including on raise_for_status() / max_size break / decode
+                # errors — otherwise the connection leaks back to the pool
+                # and long-running agents exhaust ports.
+                try:
+                    resp.raise_for_status()
 
-                # Read: unlimited if max_size is None, chunked with limit otherwise
-                if max_size is None:
-                    raw = resp.content
-                    truncated = False
-                else:
-                    chunks = []
-                    total = 0
-                    for chunk in resp.iter_content(chunk_size=65536, decode_unicode=False):
-                        if chunk:
-                            chunks.append(chunk)
-                            total += len(chunk)
-                            if total > max_size:
-                                break
-                    raw = b"".join(chunks)
-                    truncated = total > max_size
+                    # Read: unlimited if max_size is None, chunked with limit otherwise
+                    if max_size is None:
+                        raw = resp.content
+                        truncated = False
+                    else:
+                        chunks = []
+                        total = 0
+                        for chunk in resp.iter_content(chunk_size=65536, decode_unicode=False):
+                            if chunk:
+                                chunks.append(chunk)
+                                total += len(chunk)
+                                if total > max_size:
+                                    break
+                        raw = b"".join(chunks)
+                        truncated = total > max_size
 
-                if encoding:
-                    content = raw.decode(encoding)
-                else:
-                    charset = detect_charset(raw, resp.headers.get("Content-Type", ""))
-                    try:
-                        content = raw.decode(charset)
-                    except (UnicodeDecodeError, LookupError):
-                        content = raw.decode("utf-8", errors="replace")
+                    if encoding:
+                        content = raw.decode(encoding)
+                    else:
+                        charset = detect_charset(raw, resp.headers.get("Content-Type", ""))
+                        try:
+                            content = raw.decode(charset)
+                        except (UnicodeDecodeError, LookupError):
+                            content = raw.decode("utf-8", errors="replace")
 
-                return FetchResult(content, resp.headers.get("Content-Type", ""),
-                                   resp.url, truncated, ua)
+                    return FetchResult(content, resp.headers.get("Content-Type", ""),
+                                       resp.url, truncated, ua)
+                finally:
+                    resp.close()
 
             # stdlib fallback
             req = urllib.request.Request(url, headers=headers)
@@ -588,7 +595,7 @@ def fetch_url(url: str, timeout: int = 15, user_agent: str = None,
                 logger.info(f"  Fetch retry {attempt+1}/{max_retries} ({e}) in {delay:.1f}s")
                 time.sleep(delay)
                 continue
-            raise RuntimeError(f"HTTP {e.code} for {url}")
+            raise RuntimeError(f"HTTP {e.code} for {url}") from e
         except (urllib.error.URLError, OSError, TimeoutError) as e:
             last_error = e
             if attempt < max_retries:
@@ -596,7 +603,7 @@ def fetch_url(url: str, timeout: int = 15, user_agent: str = None,
                 logger.info(f"  Fetch retry {attempt+1}/{max_retries} ({e}) in {delay:.1f}s")
                 time.sleep(delay)
                 continue
-            raise RuntimeError(f"Request failed for {url}: {e}")
+            raise RuntimeError(f"Request failed for {url}: {e}") from e
         except Exception as e:
             # requests backend: retry only on connection errors (no response)
             # or transient 429/5xx; do NOT retry permanent errors like 404.
@@ -608,9 +615,9 @@ def fetch_url(url: str, timeout: int = 15, user_agent: str = None,
                     logger.info(f"  Fetch retry {attempt+1}/{max_retries} ({e}) in {delay:.1f}s")
                     time.sleep(delay)
                     continue
-            raise RuntimeError(f"Request failed for {url}: {e}")
+            raise RuntimeError(f"Request failed for {url}: {e}") from e
 
-    raise RuntimeError(f"All {max_retries+1} attempts failed for {url}: {last_error}")
+    raise RuntimeError(f"All {max_retries+1} attempts failed for {url}: {last_error}") from last_error
 
 
 # ----- Main -----