feat(routing): ordered backend candidates + real-probing doctor

- backends is now an ordered candidate list (first = preferred); channels
  report the backend actually serving via active_backend, surfaced in the
  doctor text report and --json
- new agent_reach/probe.py really executes upstream commands and tells
  apart missing / broken (stale venv shebang after a system Python
  upgrade) / timeout, with a reinstall prescription for broken installs
- all 13 channels migrated off which()-only checks: fixes bilibili
  false-positive "bili-cli 可用" on broken shims, misleading xiaohongshu
  "连接失败", rdt OSError crashing doctor, mcporter breakage masquerading
  as "未配置"
- twitter: 15s probe + 1 retry (flaky 10s timeout), broken twitter-cli
  now falls back to bird instead of aborting the check
- doctor survives per-channel exceptions; config supports per-channel
  backend override (<channel>_backend / <CHANNEL>_BACKEND env)
- fix skill install/uninstall crash on symlinked skill dirs (the
  "[Errno None] None" warning from shutil.rmtree on a symlink)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

agent_reach/channels/xueqiu.py

@@ -162,12 +162,14 @@ class XueqiuChannel(Channel):
     # ------------------------------------------------------------------ #
 
     def check(self, config=None):
+        self.active_backend = None
         try:
             data = _get_json(
                 "https://stock.xueqiu.com/v5/stock/batch/quote.json?symbol=SH000001"
             )
             items = (data.get("data") or {}).get("items") or []
             if items:
+                self.active_backend = self.backends[0]
                 return "ok", "公开 API 可用（行情、搜索、热帖、热股）"
             return "warn", "API 响应异常（返回数据为空）"
         except Exception as e: