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/bilibili.py

@@ -3,9 +3,10 @@
 
 import json
 import os
-import shutil
-import subprocess
 import urllib.request
+
+from agent_reach.probe import probe_command
+
 from .base import Channel
 
 _UA = "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36"
@@ -36,11 +37,23 @@ class BilibiliChannel(Channel):
         return "bilibili.com" in d or "b23.tv" in d
 
     def check(self, config=None):
-        if not shutil.which("yt-dlp"):
+        self.active_backend = None
+
+        # 真跑 yt-dlp --version，区分 未装 / 断链 / 异常（which 命中不等于能用）
+        yt = probe_command("yt-dlp", ["--version"], timeout=10, package="yt-dlp")
+        if yt.status == "missing":
             return "off", "yt-dlp 未安装。安装：pip install yt-dlp"
+        if yt.status == "broken":
+            return "error", "yt-dlp 已安装但无法执行\n" + yt.hint
+        if not yt.ok:
+            detail = yt.hint or yt.output or yt.status
+            return "error", f"yt-dlp 探测失败（{yt.status}）：{detail}"
+
+        self.active_backend = "yt-dlp"
 
         proxy = (config.get("bilibili_proxy") if config else None) or os.environ.get("BILIBILI_PROXY")
-        has_bili_cli = bool(shutil.which("bili"))
+        # 真跑 bili --version——断链时旧的 which 检测会误报"bili-cli 可用"
+        bili = probe_command("bili", ["--version"], timeout=10, package="bilibili-cli")
 
         parts = []
 
@@ -51,16 +64,22 @@ class BilibiliChannel(Channel):
             parts.append("视频读取：yt-dlp")
 
         # bili-cli 增强
-        if has_bili_cli:
+        if bili.ok:
             parts.append("搜索/热门/排行：bili-cli 可用")
+            status = "ok"
         else:
-            # 检测搜索 API 连通性
+            if bili.status == "broken":
+                parts.append("bili-cli 已安装但无法执行，不计为可用\n" + bili.hint)
+            elif bili.status in ("timeout", "error"):
+                parts.append(f"bili-cli 探测失败（{bili.status}），不计为可用")
+            # 降级走搜索 API；只探测一次，message 和 status 共用结果
             api_ok = _search_api_ok()
             if api_ok:
                 parts.append("搜索：B站 API 可用")
             else:
                 parts.append("搜索：B站 API 不可达")
-            parts.append("提示：安装 bili-cli 可解锁热门/排行/动态：pipx install bilibili-cli")
+            if bili.status == "missing":
+                parts.append("提示：安装 bili-cli 可解锁热门/排行/动态：pipx install bilibili-cli")
+            status = "ok" if api_ok else "warn"
 
-        status = "ok" if has_bili_cli or _search_api_ok() else "warn"
         return status, "。".join(parts)