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/rss.py

@@ -15,7 +15,13 @@ class RSSChannel(Channel):
 
     def check(self, config=None):
         try:
-            import feedparser
-            return "ok", "可读取 RSS/Atom 源"
+            import feedparser  # noqa: F401
         except ImportError:
+            self.active_backend = None
             return "off", "feedparser 未安装。安装：pip install feedparser"
+        except Exception as e:
+            # 已安装但导入期崩溃（半残安装/版本冲突）→ 重装处方
+            self.active_backend = None
+            return "error", f"feedparser 导入失败：{e}\n修复：pip install --force-reinstall feedparser"
+        self.active_backend = self.backends[0]
+        return "ok", "可读取 RSS/Atom 源"