fix: Windows encoding + .cmd subprocess resolution (#64) (#66)

- Replace text=True with encoding='utf-8', errors='replace' in all
  subprocess.run calls (channels + cli.py) to fix GBK decode errors
  on Chinese Windows systems
- Use shutil.which() resolved paths in channel subprocess calls to
  handle Windows .cmd wrappers (npm global installs)

Co-authored-by: Panniantong <panniantong@users.noreply.github.com>

agent_reach/channels/twitter.py

@@ -26,7 +26,8 @@ class TwitterChannel(Channel):
             )
         try:
             r = subprocess.run(
-                [xreach, "auth", "check"], capture_output=True, text=True, timeout=10
+                [xreach, "auth", "check"], capture_output=True,
+                encoding="utf-8", errors="replace", timeout=10
             )
             if r.returncode == 0:
                 return "ok", "完整可用（读取、搜索推文）"