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/v2ex.py

@@ -41,8 +41,10 @@ class V2EXChannel(Channel):
             _get_json(
                 "https://www.v2ex.com/api/topics/show.json?node_name=python&page=1"
             )
+            self.active_backend = self.backends[0]
             return "ok", "公开 API 可用（热门主题、节点浏览、主题详情、用户信息）"
         except Exception as e:
+            self.active_backend = None
             return "warn", f"V2EX API 连接失败（可能需要代理）：{e}"
 
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