fix(windows): force UTF-8 in channel subprocesses and MCP server env (#345)

Extracted from #318 (the UTF-8/doctor core, minus the env-wrapper feature):

- new agent_reach/utils/process.py: utf8_subprocess_env() +
  mcporter_utf8_env_args() — Windows GBK consoles otherwise corrupt
  Chinese output from mcporter/MCP child processes
- weibo/douyin/linkedin checks and weibo install/registration now pass
  the UTF-8 env (and register the MCP server with --env PYTHONUTF8=1)
- youtube: extract _has_js_runtime_config() with an OSError guard so an
  unreadable yt-dlp config can't crash doctor
- test_skill_command: open SKILL.md with explicit utf-8 (Windows GBK
  default broke these tests)

Co-authored-by: chidao <2980933590@qq.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

agent_reach/channels/youtube.py

@@ -9,6 +9,16 @@ from agent_reach.utils.text import read_utf8_text
 from .base import Channel
 
 
+def _has_js_runtime_config(config_path) -> bool:
+    """Return whether yt-dlp config explicitly enables a JS runtime."""
+    try:
+        if not config_path.exists():
+            return False
+        return "--js-runtimes" in read_utf8_text(config_path)
+    except OSError:
+        return False
+
+
 class YouTubeChannel(Channel):
     name = "youtube"
     description = "YouTube 视频和字幕"
@@ -36,10 +46,7 @@ class YouTubeChannel(Channel):
         has_deno = shutil.which("deno")
         if not has_deno:
             ytdlp_config = get_ytdlp_config_path()
-            has_js_config = False
-            if ytdlp_config.exists():
-                has_js_config = "--js-runtimes" in read_utf8_text(ytdlp_config)
-            if not has_js_config:
+            if not _has_js_runtime_config(ytdlp_config):
                 return "warn", (
                     f"yt-dlp 已安装但未配置 JS runtime。运行：\n  {render_ytdlp_fix_command()}"
                 )