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/probe.py

@@ -0,0 +1,99 @@
+# -*- coding: utf-8 -*-
+"""Lightweight upstream command probing.
+
+Distinguishes the three failure modes that look identical to shutil.which():
+  - missing: command not on PATH
+  - broken: command exists but cannot execute — most commonly a stale venv
+    shebang after a system Python upgrade (pipx/uv tool installs break this
+    way: which() finds the shim, but exec fails with FileNotFoundError
+    pointing at the shim itself)
+  - timeout/error: command runs but misbehaves
+
+Channels use probe_command() inside check() so doctor reports real health,
+not just file existence.
+"""
+
+import shutil
+import subprocess
+from dataclasses import dataclass
+from typing import Optional, Sequence
+
+from agent_reach.utils.process import utf8_subprocess_env
+
+#: Exit codes shells use for "found but not executable" / "not found".
+_BROKEN_EXIT_CODES = (126, 127)
+
+
+@dataclass
+class ProbeResult:
+    status: str  # "ok" | "missing" | "broken" | "timeout" | "error"
+    output: str = ""
+    hint: str = ""
+
+    @property
+    def ok(self) -> bool:
+        return self.status == "ok"
+
+
+def reinstall_hint(package: str) -> str:
+    """Prescription for a broken (stale-venv) CLI install."""
+    return (
+        f"命令存在但无法执行——通常是系统 Python 升级后 venv 解释器丢失。重装即可修复：\n"
+        f"  uv tool install --force {package}\n"
+        f"或：pipx reinstall {package}"
+    )
+
+
+def probe_command(
+    cmd: str,
+    args: Sequence[str] = ("--version",),
+    timeout: int = 10,
+    retries: int = 0,
+    package: Optional[str] = None,
+) -> ProbeResult:
+    """Actually execute `cmd *args` and classify the result.
+
+    package: pip/pipx package name used in the broken-install hint
+             (defaults to cmd).
+    """
+    path = shutil.which(cmd)
+    if not path:
+        return ProbeResult("missing")
+
+    last: Optional[ProbeResult] = None
+    for _ in range(retries + 1):
+        last = _run_once(path, args, timeout, package or cmd)
+        if last.ok:
+            return last
+        # missing/broken won't heal between retries — only transient
+        # failures (timeout/error) are worth a second attempt
+        if last.status in ("missing", "broken"):
+            return last
+    return last
+
+
+def _run_once(path: str, args: Sequence[str], timeout: int, package: str) -> ProbeResult:
+    try:
+        r = subprocess.run(
+            [path, *args],
+            capture_output=True,
+            encoding="utf-8",
+            errors="replace",
+            timeout=timeout,
+            env=utf8_subprocess_env(),
+        )
+    except FileNotFoundError:
+        # which() found it but exec failed: the shebang interpreter is gone
+        return ProbeResult("broken", hint=reinstall_hint(package))
+    except OSError:
+        return ProbeResult("broken", hint=reinstall_hint(package))
+    except subprocess.TimeoutExpired:
+        return ProbeResult("timeout", hint=f"`{path}` 响应超时（>{timeout}s）")
+
+    if r.returncode in _BROKEN_EXIT_CODES:
+        return ProbeResult("broken", hint=reinstall_hint(package))
+
+    output = (r.stdout or "") + (r.stderr or "")
+    if r.returncode != 0:
+        return ProbeResult("error", output=output.strip())
+    return ProbeResult("ok", output=output.strip())