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/github.py

@@ -1,8 +1,8 @@
 # -*- coding: utf-8 -*-
 """GitHub — check if gh CLI is available."""
 
-import shutil
-import subprocess
+from agent_reach.probe import probe_command
+
 from .base import Channel
 
 
@@ -17,16 +17,26 @@ class GitHubChannel(Channel):
         return "github.com" in urlparse(url).netloc.lower()
 
     def check(self, config=None):
-        gh = shutil.which("gh")
-        if not gh:
+        # 真跑 gh auth status 探活。注意：未登录时 rc!=0 是正常业务态（warn），不是 error。
+        probe = probe_command("gh", ["auth", "status"], timeout=10, package="gh")
+        if probe.status == "missing":
+            self.active_backend = None
             return "warn", "gh CLI 未安装。安装：https://cli.github.com"
-        try:
-            r = subprocess.run(
-                [gh, "auth", "status"],
-                capture_output=True, encoding="utf-8", errors="replace", timeout=5
+        if probe.status == "broken":
+            # gh 是二进制安装（brew/官方包），不是 pip 包——处方不用 pipx/uv 文案
+            self.active_backend = None
+            return "error", (
+                "gh 命令存在但无法执行——安装已损坏。重装即可修复：\n"
+                "  brew reinstall gh\n"
+                "或从 https://cli.github.com 重新安装 gh CLI"
             )
-            if r.returncode == 0:
-                return "ok", "完整可用（读取、搜索、Fork、Issue、PR 等）"
-            return "warn", "gh CLI 已安装但未认证。运行 gh auth login 可解锁完整功能"
-        except Exception:
-            return "warn", "gh CLI 状态检查失败，运行 gh auth status 查看详情"
+        if probe.status == "timeout":
+            # gh 本体能启动（工具是活的），只是状态检查超时
+            self.active_backend = "gh CLI"
+            return "warn", "gh CLI 状态检查超时，运行 gh auth status 查看详情"
+        if probe.ok:
+            self.active_backend = "gh CLI"
+            return "ok", "完整可用（读取、搜索、Fork、Issue、PR 等）"
+        # rc != 0：gh 活着但未认证（gh auth status 的正常业务态）
+        self.active_backend = "gh CLI"
+        return "warn", "gh CLI 已安装但未认证。运行 gh auth login 可解锁完整功能"