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/cli.py

@@ -364,8 +364,11 @@ def _install_skill():
     def _copy_skill_dir(target: str) -> bool:
         """Copy entire skill directory (locale-specific SKILL.md + references/)."""
         try:
-            # Clear existing installation
-            if os.path.exists(target):
+            # Clear existing installation. A symlinked skill dir (dotfiles
+            # setups) breaks shutil.rmtree — unlink the link itself instead.
+            if os.path.islink(target):
+                os.unlink(target)
+            elif os.path.exists(target):
                 shutil.rmtree(target)
             os.makedirs(target, exist_ok=True)
 
@@ -454,7 +457,10 @@ def _uninstall_skill():
         skill_path = os.path.expanduser(skill_path_template)
         if os.path.isdir(skill_path):
             try:
-                shutil.rmtree(skill_path)
+                if os.path.islink(skill_path):
+                    os.unlink(skill_path)
+                else:
+                    shutil.rmtree(skill_path)
                 print(f"  Removed {platform_name} skill: {skill_path}")
                 removed = True
             except Exception as e: