fix: replace 'mcporter list' with 'mcporter config list' to prevent hangs

mcporter list probes all configured servers sequentially, which can hang
indefinitely when servers are slow/offline. mcporter config list reads
config without probing — sufficient for checking if a server is configured.

Affected: doctor checks (4 channels) and install command (2 calls).

Fixes #34

agent_reach/channels/exa_search.py

@@ -24,7 +24,7 @@ class ExaSearchChannel(Channel):
             )
         try:
             r = subprocess.run(
-                ["mcporter", "list"], capture_output=True, text=True, timeout=10
+                ["mcporter", "config", "list"], capture_output=True, text=True, timeout=5
             )
             if "exa" in r.stdout.lower():
                 return "ok", "全网语义搜索可用（免费，无需 API Key）"