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>

tests/test_process.py

@@ -0,0 +1,18 @@
+from agent_reach.utils.process import mcporter_utf8_env_args, utf8_subprocess_env
+
+
+def test_utf8_subprocess_env_forces_python_utf8():
+    env = utf8_subprocess_env({"PYTHONUTF8": "0", "OTHER": "value"})
+
+    assert env["PYTHONUTF8"] == "1"
+    assert env["PYTHONIOENCODING"] == "utf-8"
+    assert env["OTHER"] == "value"
+
+
+def test_mcporter_utf8_env_args():
+    assert mcporter_utf8_env_args() == [
+        "--env",
+        "PYTHONUTF8=1",
+        "--env",
+        "PYTHONIOENCODING=utf-8",
+    ]