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/watch.py

@@ -177,7 +177,7 @@ def main() -> int:
         mins = int(full_duration // 60)
         print()
         print(
-            f"> ⚠️ This is a {mins}-minute video. Frame coverage is sparse at this length — "
+            f"> **Warning:** This is a {mins}-minute video. Frame coverage is sparse at this length — "
             "accuracy degrades noticeably on anything over 10 minutes. For better results, "
             "re-run with `--start HH:MM:SS --end HH:MM:SS` to zoom into a specific section."
         )