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_twitter_channel.py

@@ -26,6 +26,7 @@ def test_check_twitter_cli_found_and_auth_ok():
     assert status == "ok"
     assert "twitter-cli" in message
     assert "完整可用" in message
+    assert channel.active_backend == "twitter-cli"
 
 
 def test_check_twitter_cli_found_auth_missing():
@@ -41,6 +42,8 @@ def test_check_twitter_cli_found_auth_missing():
         status, message = channel.check()
     assert status == "warn"
     assert "未认证" in message
+    # 未认证是业务态：工具进程活着，后端仍可用
+    assert channel.active_backend == "twitter-cli"
 
 
 # --- bird CLI fallback tests ---
@@ -59,6 +62,7 @@ def test_check_bird_fallback_auth_ok():
         status, message = channel.check()
     assert status == "ok"
     assert "bird" in message
+    assert channel.active_backend == "bird CLI (legacy)"
 
 
 def test_check_bird_fallback_auth_missing():
@@ -86,6 +90,7 @@ def test_check_nothing_installed():
         status, message = channel.check()
     assert status == "warn"
     assert "twitter-cli" in message
+    assert channel.active_backend is None
 
 
 # --- twitter-cli preferred over bird ---
@@ -106,3 +111,44 @@ def test_twitter_cli_preferred_over_bird():
         status, message = channel.check()
     assert status == "ok"
     assert "twitter-cli" in message
+    assert channel.active_backend == "twitter-cli"
+
+
+# --- broken install (stale venv shim) ---
+
+def test_check_twitter_cli_broken_reports_error_with_reinstall_hint():
+    """which 命中但 exec 抛 FileNotFoundError（venv 断链）→ error + 重装处方。"""
+    channel = TwitterChannel()
+    with patch(
+        "shutil.which",
+        side_effect=lambda name: "/usr/local/bin/twitter" if name == "twitter" else None,
+    ), patch("subprocess.run", side_effect=FileNotFoundError("/usr/local/bin/twitter")):
+        status, message = channel.check()
+    assert status == "error"
+    assert "无法执行" in message
+    assert "uv tool install --force twitter-cli" in message
+    assert "pipx reinstall twitter-cli" in message
+    assert channel.active_backend is None
+
+
+def test_check_twitter_cli_broken_falls_back_to_bird():
+    """twitter-cli 断链但 bird 健康 → 回退到 bird，后端正确归属。"""
+    channel = TwitterChannel()
+
+    def which_side_effect(name):
+        if name in ("twitter", "bird"):
+            return f"/usr/local/bin/{name}"
+        return None
+
+    def run_side_effect(cmd, **kwargs):
+        if "twitter" in cmd[0]:
+            raise FileNotFoundError(cmd[0])
+        return _cp(stdout="Authenticated as @user\n", returncode=0)
+
+    with patch("shutil.which", side_effect=which_side_effect), patch(
+        "subprocess.run", side_effect=run_side_effect
+    ):
+        status, message = channel.check()
+    assert status == "ok"
+    assert "bird" in message
+    assert channel.active_backend == "bird CLI (legacy)"