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/linkedin.py

@@ -1,13 +1,13 @@
 # -*- coding: utf-8 -*-
 """LinkedIn — check if linkedin-scraper-mcp is available."""
 
-import shutil
-import subprocess
-
-from agent_reach.utils.process import utf8_subprocess_env
+from agent_reach.probe import probe_command
 
 from .base import Channel
 
+#: mcporter 是 npm 包，断链处方与默认的 pipx/uv 不同
+_MCPORTER_BROKEN_HINT = "mcporter 无法执行（node 环境损坏），重装：\n  npm install -g mcporter"
+
 
 class LinkedInChannel(Channel):
     name = "linkedin"
@@ -20,24 +20,22 @@ class LinkedInChannel(Channel):
         return "linkedin.com" in urlparse(url).netloc.lower()
 
     def check(self, config=None):
-        mcporter = shutil.which("mcporter")
-        if not mcporter:
+        self.active_backend = None
+        probe = probe_command("mcporter", ["config", "list"], timeout=10, package="mcporter")
+        if probe.status == "missing":
             return "off", (
                 "基本内容可通过 Jina Reader 读取。完整功能需要：\n"
                 "  pip install linkedin-scraper-mcp\n"
                 "  mcporter config add linkedin http://localhost:3000/mcp\n"
                 "  详见 https://github.com/stickerdaniel/linkedin-mcp-server"
             )
-        try:
-            r = subprocess.run(
-                [mcporter, "config", "list"], capture_output=True,
-                encoding="utf-8", errors="replace", timeout=5,
-                env=utf8_subprocess_env(),
-            )
-            if "linkedin" in r.stdout.lower():
-                return "ok", "完整可用（Profile、公司、职位搜索）"
-        except Exception:
-            pass
+        if probe.status == "broken":
+            return "error", _MCPORTER_BROKEN_HINT
+        if not probe.ok:  # timeout / error
+            return "error", f"mcporter 执行异常：{probe.hint or probe.output or probe.status}"
+        if "linkedin" in probe.output.lower():
+            self.active_backend = "linkedin-scraper-mcp"
+            return "ok", "完整可用（Profile、公司、职位搜索）"
         return "off", (
             "mcporter 已装但 LinkedIn MCP 未配置。运行：\n"
             "  pip install linkedin-scraper-mcp\n"