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>

tests/test_doctor.py

@@ -8,13 +8,15 @@ from agent_reach.config import Config
 
 
 class _StubChannel:
-    def __init__(self, name, description, tier, status, message, backends=None):
+    def __init__(self, name, description, tier, status, message, backends=None,
+                 active_backend=None):
         self.name = name
         self.description = description
         self.tier = tier
         self._status = status
         self._message = message
         self.backends = backends or []
+        self.active_backend = active_backend
 
     def check(self, config=None):
         return self._status, self._message
@@ -31,7 +33,8 @@ class TestDoctor:
             doctor,
             "get_all_channels",
             lambda: [
-                _StubChannel("web", "网页", 0, "ok", "可抓取网页", ["requests"]),
+                _StubChannel("web", "网页", 0, "ok", "可抓取网页", ["requests"],
+                             active_backend="requests"),
                 _StubChannel("github", "GitHub", 0, "warn", "gh 未安装", ["gh"]),
                 _StubChannel("exa_search", "全网语义搜索", 1, "off", "mcporter 未配置", ["Exa"]),
             ],
@@ -46,6 +49,7 @@ class TestDoctor:
                 "message": "可抓取网页",
                 "tier": 0,
                 "backends": ["requests"],
+                "active_backend": "requests",
             },
             "github": {
                 "status": "warn",
@@ -53,6 +57,7 @@ class TestDoctor:
                 "message": "gh 未安装",
                 "tier": 0,
                 "backends": ["gh"],
+                "active_backend": None,
             },
             "exa_search": {
                 "status": "off",
@@ -60,6 +65,7 @@ class TestDoctor:
                 "message": "mcporter 未配置",
                 "tier": 1,
                 "backends": ["Exa"],
+                "active_backend": None,
             },
         }