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/doctor.py

@@ -10,20 +10,37 @@ from agent_reach.channels import get_all_channels
 
 
 def check_all(config: Config) -> Dict[str, dict]:
-    """Check all channels and return status dict."""
+    """Check all channels and return status dict.
+
+    A single misbehaving channel must never take the whole report down,
+    so per-channel exceptions degrade to status="error".
+    """
     results = {}
     for ch in get_all_channels():
-        status, message = ch.check(config)
+        try:
+            status, message = ch.check(config)
+        except Exception as e:  # noqa: BLE001 — doctor must survive any channel
+            status, message = "error", f"体检异常：{e}"
         results[ch.name] = {
             "status": status,
             "name": ch.description,
             "message": message,
             "tier": ch.tier,
             "backends": ch.backends,
+            "active_backend": getattr(ch, "active_backend", None),
         }
     return results
 
 
+def _name_msg(r: dict, escape) -> str:
+    """Render one channel line; show the active backend when there is a choice."""
+    text = f"[bold]{escape(r['name'])}[/bold] — {escape(r['message'])}"
+    active = r.get("active_backend")
+    if active and len(r.get("backends", [])) > 1:
+        text += f" [dim]（当前后端：{escape(active)}）[/dim]"
+    return text
+
+
 def format_report(results: Dict[str, dict]) -> str:
     """Format results as a readable text report (with Rich markup)."""
     try:
@@ -44,7 +61,7 @@ def format_report(results: Dict[str, dict]) -> str:
     lines.append("[bold]✅ 装好即用：[/bold]")
     for key, r in results.items():
         if r["tier"] == 0:
-            name_msg = f"[bold]{escape(r['name'])}[/bold] — {escape(r['message'])}"
+            name_msg = _name_msg(r, escape)
             if r["status"] == "ok":
                 lines.append(f"  [green]✅[/green] {name_msg}")
             elif r["status"] == "warn":
@@ -60,8 +77,7 @@ def format_report(results: Dict[str, dict]) -> str:
         lines.append("")
         lines.append("[bold]可选渠道（已安装）：[/bold]")
         for key, r in tier1_active.items():
-            name_msg = f"[bold]{escape(r['name'])}[/bold] — {escape(r['message'])}"
-            lines.append(f"  [green]✅[/green] {name_msg}")
+            lines.append(f"  [green]✅[/green] {_name_msg(r, escape)}")
 
     # Tier 2 — optional complex setup
     tier2 = {k: r for k, r in results.items() if r["tier"] == 2}
@@ -72,8 +88,7 @@ def format_report(results: Dict[str, dict]) -> str:
             lines.append("")
             lines.append("[bold]可选渠道（已安装）：[/bold]")
         for key, r in tier2_active.items():
-            name_msg = f"[bold]{escape(r['name'])}[/bold] — {escape(r['message'])}"
-            lines.append(f"  [green]✅[/green] {name_msg}")
+            lines.append(f"  [green]✅[/green] {_name_msg(r, escape)}")
 
     lines.append("")
     status_color = "green" if ok_count == total else ("yellow" if ok_count > 0 else "red")