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_channel_contracts.py

@@ -1,10 +1,17 @@
 # -*- coding: utf-8 -*-
 """Contract tests for channel adapters."""
 
+import subprocess
+
 from agent_reach.channels import get_all_channels
 from agent_reach.config import Config
 
 
+def _fake_run_ok(cmd, **kwargs):
+    """Pretend any probed CLI executes fine and prints a version."""
+    return subprocess.CompletedProcess(cmd, 0, "2026.06.09", "")
+
+
 def test_channel_registry_contract():
     channels = get_all_channels()
     assert channels, "channel registry must not be empty"
@@ -29,6 +36,67 @@ def test_channel_check_contract_with_minimal_runtime(monkeypatch, tmp_path):
         assert isinstance(message, str) and message.strip()
 
 
+def test_channel_active_backend_attribute_contract():
+    """Every channel exposes active_backend (default None, or str once set)."""
+    for ch in get_all_channels():
+        assert hasattr(ch, "active_backend")
+        # Fresh instances must default to None / str (class attribute on base)
+        fresh = type(ch)()
+        assert fresh.active_backend is None or isinstance(fresh.active_backend, str)
+
+
+def test_channel_active_backend_set_by_check(monkeypatch, tmp_path):
+    """After check(), active_backend is None or a str — never anything else."""
+    monkeypatch.setattr("shutil.which", lambda _cmd: None)
+
+    # Keep the network-based channels (V2EX/Xueqiu/Bilibili API) deterministic.
+    import urllib.request
+    from urllib.error import URLError
+
+    def _no_net(*_a, **_k):
+        raise URLError("offline")
+
+    monkeypatch.setattr(urllib.request, "urlopen", _no_net)
+    import agent_reach.channels.xueqiu as xueqiu_mod
+    monkeypatch.setattr(xueqiu_mod, "_cookies_initialized", True)
+    monkeypatch.setattr(xueqiu_mod._opener, "open", _no_net)
+
+    config = Config(config_path=tmp_path / "config.yaml")
+    for ch in get_all_channels():
+        ch.check(config)
+        assert ch.active_backend is None or isinstance(ch.active_backend, str), (
+            f"{ch.name}: active_backend must be None or str after check()"
+        )
+
+
+def test_ordered_backends_contract(tmp_path):
+    """ordered_backends(config) is a reordering (same multiset) of backends."""
+    config = Config(config_path=tmp_path / "config.yaml")
+    for ch in get_all_channels():
+        ordered = ch.ordered_backends(config)
+        assert isinstance(ordered, list)
+        assert sorted(ordered) == sorted(ch.backends), (
+            f"{ch.name}: ordered_backends must be a permutation of backends"
+        )
+        # And without any config at all
+        ordered_none = ch.ordered_backends(None)
+        assert sorted(ordered_none) == sorted(ch.backends)
+
+
+def test_ordered_backends_override_moves_backend_to_front():
+    """Config key <channel>_backend promotes the named backend to front."""
+    from agent_reach.channels.twitter import TwitterChannel
+
+    ch = TwitterChannel()
+    ordered = ch.ordered_backends({"twitter_backend": "bird"})
+    assert ordered[0] == "bird CLI (legacy)"
+    assert sorted(ordered) == sorted(ch.backends)
+
+    # Unknown override is ignored — never hides working backends
+    ordered_unknown = ch.ordered_backends({"twitter_backend": "no-such-tool"})
+    assert ordered_unknown == list(ch.backends)
+
+
 def test_youtube_warns_when_node_only_and_no_config(monkeypatch, tmp_path):
     """YouTube should warn when only Node.js is installed but no yt-dlp config exists."""
     from agent_reach.channels.youtube import YouTubeChannel
@@ -41,6 +109,7 @@ def test_youtube_warns_when_node_only_and_no_config(monkeypatch, tmp_path):
         return None  # deno not installed
 
     monkeypatch.setattr("shutil.which", fake_which)
+    monkeypatch.setattr("subprocess.run", _fake_run_ok)  # yt-dlp probe really executes now
     # Point to a non-existent config file
     monkeypatch.setattr("os.path.expanduser", lambda p: str(tmp_path / ".config/yt-dlp/config"))
 
@@ -48,6 +117,7 @@ def test_youtube_warns_when_node_only_and_no_config(monkeypatch, tmp_path):
     status, message = ch.check()
     assert status == "warn"
     assert "--js-runtimes" in message
+    assert ch.active_backend == "yt-dlp"  # 本体活着，warn 只关乎 JS runtime
 
 
 def test_youtube_warns_with_windows_specific_fix_command(monkeypatch, tmp_path):
@@ -62,6 +132,7 @@ def test_youtube_warns_with_windows_specific_fix_command(monkeypatch, tmp_path):
         return None
 
     monkeypatch.setattr("shutil.which", fake_which)
+    monkeypatch.setattr("subprocess.run", _fake_run_ok)  # yt-dlp probe really executes now
     monkeypatch.setattr("agent_reach.utils.paths.sys.platform", "win32")
     monkeypatch.setenv("APPDATA", str(tmp_path / "AppData" / "Roaming"))
 
@@ -84,10 +155,12 @@ def test_youtube_ok_when_deno_installed(monkeypatch):
         return None
 
     monkeypatch.setattr("shutil.which", fake_which)
+    monkeypatch.setattr("subprocess.run", _fake_run_ok)  # yt-dlp probe really executes now
 
     ch = YouTubeChannel()
     status, _msg = ch.check()
     assert status == "ok"
+    assert ch.active_backend == "yt-dlp"
 
 
 def test_channel_can_handle_contract():