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_probe.py

@@ -0,0 +1,90 @@
+# -*- coding: utf-8 -*-
+"""Tests for agent_reach.probe — real-execution probing and failure classification."""
+
+import os
+import stat
+import sys
+
+import pytest
+
+from agent_reach.probe import ProbeResult, probe_command, reinstall_hint
+
+
+def _make_executable(path, content):
+    path.write_text(content)
+    path.chmod(path.stat().st_mode | stat.S_IXUSR | stat.S_IXGRP | stat.S_IXOTH)
+    return str(path)
+
+
+def test_missing_command():
+    r = probe_command("definitely-not-a-real-command-xyz")
+    assert r.status == "missing"
+    assert not r.ok
+
+
+@pytest.mark.skipif(sys.platform == "win32", reason="shebang semantics are POSIX-only")
+def test_broken_shebang_detected_as_broken(tmp_path, monkeypatch):
+    """A stale venv shim: which() finds it, exec raises FileNotFoundError."""
+    script = _make_executable(
+        tmp_path / "stale-tool", "#!/nonexistent/venv/bin/python\nprint('hi')\n"
+    )
+    monkeypatch.setenv("PATH", str(tmp_path) + os.pathsep + os.environ.get("PATH", ""))
+
+    r = probe_command("stale-tool", package="stale-tool-pkg")
+    assert r.status == "broken"
+    assert "uv tool install --force stale-tool-pkg" in r.hint
+    assert "pipx reinstall stale-tool-pkg" in r.hint
+
+
+@pytest.mark.skipif(sys.platform == "win32", reason="shell script fixture is POSIX-only")
+def test_healthy_command_returns_ok_with_output(tmp_path, monkeypatch):
+    script = _make_executable(
+        tmp_path / "healthy-tool", "#!/bin/sh\necho 'healthy-tool 1.2.3'\n"
+    )
+    monkeypatch.setenv("PATH", str(tmp_path) + os.pathsep + os.environ.get("PATH", ""))
+
+    r = probe_command("healthy-tool")
+    assert r.ok
+    assert "1.2.3" in r.output
+
+
+@pytest.mark.skipif(sys.platform == "win32", reason="shell script fixture is POSIX-only")
+def test_nonzero_exit_classified_as_error(tmp_path, monkeypatch):
+    script = _make_executable(
+        tmp_path / "failing-tool", "#!/bin/sh\necho 'boom' >&2\nexit 3\n"
+    )
+    monkeypatch.setenv("PATH", str(tmp_path) + os.pathsep + os.environ.get("PATH", ""))
+
+    r = probe_command("failing-tool")
+    assert r.status == "error"
+    assert "boom" in r.output
+
+
+@pytest.mark.skipif(sys.platform == "win32", reason="shell script fixture is POSIX-only")
+def test_exit_127_classified_as_broken(tmp_path, monkeypatch):
+    script = _make_executable(tmp_path / "wrapper-tool", "#!/bin/sh\nexit 127\n")
+    monkeypatch.setenv("PATH", str(tmp_path) + os.pathsep + os.environ.get("PATH", ""))
+
+    r = probe_command("wrapper-tool", package="wrapper-pkg")
+    assert r.status == "broken"
+    assert "wrapper-pkg" in r.hint
+
+
+@pytest.mark.skipif(sys.platform == "win32", reason="shell script fixture is POSIX-only")
+def test_retries_help_transient_failures(tmp_path, monkeypatch):
+    """First call fails (exit 1), second succeeds — retries=1 should return ok."""
+    marker = tmp_path / "ran-once"
+    script = _make_executable(
+        tmp_path / "flaky-tool",
+        f"#!/bin/sh\nif [ -f {marker} ]; then echo ok; exit 0; fi\ntouch {marker}\nexit 1\n",
+    )
+    monkeypatch.setenv("PATH", str(tmp_path) + os.pathsep + os.environ.get("PATH", ""))
+
+    r = probe_command("flaky-tool", retries=1)
+    assert r.ok
+
+
+def test_reinstall_hint_mentions_both_installers():
+    hint = reinstall_hint("some-pkg")
+    assert "uv tool install --force some-pkg" in hint
+    assert "pipx reinstall some-pkg" in hint