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/youtube.py

@@ -3,6 +3,7 @@
 
 import shutil
 
+from agent_reach.probe import probe_command
 from agent_reach.utils.paths import get_ytdlp_config_path, render_ytdlp_fix_command
 from agent_reach.utils.text import read_utf8_text
 
@@ -32,8 +33,20 @@ class YouTubeChannel(Channel):
         return "youtube.com" in d or "youtu.be" in d
 
     def check(self, config=None):
-        if not shutil.which("yt-dlp"):
+        # 真跑 yt-dlp --version 探活，区分未装 / venv 断链 / 跑不动
+        probe = probe_command("yt-dlp", ["--version"], timeout=10, package="yt-dlp")
+        if probe.status == "missing":
+            self.active_backend = None
             return "off", "yt-dlp 未安装。安装：pip install yt-dlp"
+        if probe.status == "broken":
+            self.active_backend = None
+            return "error", f"yt-dlp 已安装但无法执行\n{probe.hint}"
+        if not probe.ok:  # timeout / error：装了但跑不动
+            self.active_backend = None
+            detail = probe.hint or probe.output or probe.status
+            return "error", f"yt-dlp 无法正常运行：{detail}"
+        # yt-dlp 本体是活的；后面的 JS runtime/转写检查只影响 ok/warn，不影响后端归属
+        self.active_backend = "yt-dlp"
         # Check JS runtime
         has_js = shutil.which("deno") or shutil.which("node")
         if not has_js: