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/cli.py

@@ -419,6 +419,25 @@ def _install_system_deps():
             except Exception:
                 print("  ⬜ undici install failed (optional — xreach may not work behind proxies)")
 
+    # ── yt-dlp JS runtime config (YouTube requires external JS runtime) ──
+    if shutil.which("node"):
+        ytdlp_config_dir = os.path.expanduser("~/.config/yt-dlp")
+        ytdlp_config = os.path.join(ytdlp_config_dir, "config")
+        needs_config = True
+        if os.path.exists(ytdlp_config):
+            with open(ytdlp_config, "r") as f:
+                if "--js-runtimes" in f.read():
+                    needs_config = False
+                    print("  ✅ yt-dlp JS runtime already configured")
+        if needs_config:
+            try:
+                os.makedirs(ytdlp_config_dir, exist_ok=True)
+                with open(ytdlp_config, "a") as f:
+                    f.write("--js-runtimes node\n")
+                print("  ✅ yt-dlp configured to use Node.js as JS runtime (YouTube)")
+            except Exception:
+                print("  ⬜ Could not configure yt-dlp JS runtime (YouTube may not work)")
+
 
 def _install_system_deps_safe():
     """Safe mode: check what's installed, print instructions for what's missing."""