Fix Windows compatibility: encoding, installer hints, interpreter name

- watch.py: drop the ⚠️ emoji from the long-video warning; cp1252
  consoles on Windows couldn't encode it and the script crashed.
- setup.py: add a Windows branch that prints winget / pip install
  commands, matching what the README already promised.
- SKILL.md: note that Windows users must invoke the scripts with
  `python`, not `python3` (which is the Microsoft Store stub).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/setup.py

@@ -186,6 +186,16 @@ def _install_hint_linux(missing: list[str]) -> str:
     return "\n  ".join(hints) if hints else "nothing to install"
 
 
+def _install_hint_windows(missing: list[str]) -> str:
+    pkgs = _brew_pkg(missing)
+    hints = []
+    if "ffmpeg" in pkgs:
+        hints.append("winget: `winget install Gyan.FFmpeg`")
+    if "yt-dlp" in pkgs:
+        hints.append("winget: `winget install yt-dlp.yt-dlp` or pip: `pip install --user yt-dlp`")
+    return "\n  ".join(hints) if hints else "nothing to install"
+
+
 def _status() -> dict:
     """Structured preflight snapshot."""
     missing = _check_binaries()
@@ -268,6 +278,10 @@ def cmd_install() -> int:
             print("[setup] dependencies missing on Linux — please install:", file=sys.stderr)
             print("  " + _install_hint_linux(missing), file=sys.stderr)
             return 2
+        elif system == "Windows":
+            print("[setup] dependencies missing on Windows — please install:", file=sys.stderr)
+            print("  " + _install_hint_windows(missing), file=sys.stderr)
+            return 2
         else:
             print(f"[setup] unsupported platform ({system}) for auto-install. Install manually:", file=sys.stderr)
             print(f"  missing: {', '.join(missing)}", file=sys.stderr)