fix: YouTube requires JS runtime — auto-configure Node.js in install

yt-dlp now requires an external JS runtime (deno or Node.js) for YouTube.
Since agent-reach already installs Node.js, we configure yt-dlp to use it:

1. cli.py: install writes '--js-runtimes node' to ~/.config/yt-dlp/config
2. youtube.py: doctor checks JS runtime availability and config
3. SKILL.md: updated YouTube section with accurate troubleshooting

Bilibili note: 412 errors are IP-based (needs proxy), not cookie-related.

Fixes #47

agent_reach/channels/youtube.py

@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
-"""YouTube — check if yt-dlp is available."""
+"""YouTube — check if yt-dlp is available with JS runtime."""
 
+import os
 import shutil
 from .base import Channel
 
@@ -17,6 +18,23 @@ class YouTubeChannel(Channel):
         return "youtube.com" in d or "youtu.be" in d
 
     def check(self, config=None):
-        if shutil.which("yt-dlp"):
-            return "ok", "可提取视频信息和字幕"
-        return "off", "yt-dlp 未安装。安装：pip install yt-dlp"
+        if not shutil.which("yt-dlp"):
+            return "off", "yt-dlp 未安装。安装：pip install yt-dlp"
+        # Check JS runtime
+        has_js = shutil.which("deno") or shutil.which("node")
+        if not has_js:
+            return "warn", (
+                "yt-dlp 已安装但缺少 JS runtime（YouTube 必须）。\n"
+                "  安装 Node.js 或 deno，然后运行：agent-reach install"
+            )
+        # Check yt-dlp config for --js-runtimes
+        ytdlp_config = os.path.expanduser("~/.config/yt-dlp/config")
+        has_deno = shutil.which("deno")
+        if not has_deno and os.path.exists(ytdlp_config):
+            with open(ytdlp_config, "r") as f:
+                if "--js-runtimes" not in f.read():
+                    return "warn", (
+                        "yt-dlp 已安装但未配置 JS runtime。运行：\n"
+                        "  mkdir -p ~/.config/yt-dlp && echo '--js-runtimes node' >> ~/.config/yt-dlp/config"
+                    )
+        return "ok", "可提取视频信息和字幕"