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/base.py

@@ -8,11 +8,22 @@ and provides:
   - check(config) → is the upstream tool installed and configured?
 
 After installation, agents call upstream tools directly.
+
+Backend routing semantics:
+  - `backends` is an ORDERED candidate list: backends[0] is the preferred
+    backend, the rest are fallbacks. "Switching backends" for a platform
+    means reordering this list (or a user override) — not rewriting code.
+  - check() must set `self.active_backend` to the backend that is actually
+    serving the channel right now (None when nothing usable is found).
+    shutil.which() alone is NOT proof of health — a stale venv shim passes
+    which() but cannot execute (see agent_reach.probe). Channels should
+    really execute a lightweight command before claiming a backend active.
+  - Users can force a backend with config key `<channel>_backend`
+    (or env var `<CHANNEL>_BACKEND`); ordered_backends() applies it.
 """
 
-import shutil
 from abc import ABC, abstractmethod
-from typing import List, Tuple
+from typing import List, Optional, Tuple
 
 
 class Channel(ABC):
@@ -20,17 +31,40 @@ class Channel(ABC):
 
     name: str = ""                    # e.g. "youtube"
     description: str = ""             # e.g. "YouTube 视频和字幕"
-    backends: List[str] = []          # e.g. ["yt-dlp"] — what upstream tool is used
+    backends: List[str] = []          # ordered candidates — backends[0] = preferred
     tier: int = 0                     # 0=zero-config, 1=needs free key, 2=needs setup
 
+    #: Backend currently serving this channel; set by check(), None = unavailable.
+    active_backend: Optional[str] = None
+
     @abstractmethod
     def can_handle(self, url: str) -> bool:
         """Check if this channel can handle this URL."""
         ...
 
+    def ordered_backends(self, config=None) -> List[str]:
+        """Candidate backends in probe order, honoring the user override.
+
+        The config key `<channel>_backend` (env `<CHANNEL>_BACKEND`) moves the
+        named backend to the front of the list; unknown values are ignored so
+        a stale override can never hide working backends.
+        """
+        candidates = list(self.backends)
+        override = config.get(f"{self.name}_backend") if config else None
+        if override:
+            for i, b in enumerate(candidates):
+                if b == override or b.startswith(override):
+                    candidates.insert(0, candidates.pop(i))
+                    break
+        return candidates
+
     def check(self, config=None) -> Tuple[str, str]:
         """
         Check if this channel's upstream tool is available.
         Returns (status, message) where status is 'ok'/'warn'/'off'/'error'.
+
+        Subclasses with external backends must really probe them (see
+        agent_reach.probe.probe_command) and set self.active_backend.
         """
+        self.active_backend = self.backends[0] if self.backends else "内置"
         return "ok", f"{'、'.join(self.backends) if self.backends else '内置'}"