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/linkedin.py

@@ -3,6 +3,9 @@
 
 import shutil
 import subprocess
+
+from agent_reach.utils.process import utf8_subprocess_env
+
 from .base import Channel
 
 
@@ -28,7 +31,8 @@ class LinkedInChannel(Channel):
         try:
             r = subprocess.run(
                 [mcporter, "config", "list"], capture_output=True,
-                encoding="utf-8", errors="replace", timeout=5
+                encoding="utf-8", errors="replace", timeout=5,
+                env=utf8_subprocess_env(),
             )
             if "linkedin" in r.stdout.lower():
                 return "ok", "完整可用（Profile、公司、职位搜索）"