Fix Windows compatibility: UTF-8 stdout, pip self-upgrade, complete UA

[chewinggum604-art]

Summary

This PR fixes three Windows-specific issues that block first-run setup of the
skill on a fresh install. Tested on Windows 11 with Python 3.12.10 (winget,
user scope) and Chrome 131. No behavior change on macOS/Linux.

What's broken on Windows today

1. scripts/run.py crashes immediately on first run.
   The runner prints a 🔧 emoji during venv bootstrap. On Windows the default
   console encoding is cp1252, which can't encode \U0001f527. Result:
   UnicodeEncodeError before any work happens.

2. scripts/setup_environment.py fails the dependency install.
   It upgrades pip via pip.exe install --upgrade pip. Windows can't replace
   pip.exe while it's running (FileLock), so the call returns non-zero,
   check=True raises, and ensure_venv() returns False. Setup aborts.

3. scripts/config.py ships a truncated USER_AGENT missing the
   (KHTML, like Gecko) Chrome/<ver> Safari/537.36 tail. Truncated UAs are a
   bot-detection signal — minor, but cheap to fix.

Changes

• scripts/run.py

  • Reconfigure sys.stdout / sys.stderr to UTF-8 with errors='replace'
    at the top of the file (Python 3.7+).
  • Set PYTHONIOENCODING=utf-8 in the env passed to subprocess calls so the
    fix propagates to setup_environment.py, auth_manager.py,
    notebook_manager.py, and ask_question.py when launched via the runner.

• scripts/setup_environment.py

  • Switch the pip self-upgrade from [venv_pip, "install", ...] to
    [venv_python, "-m", "pip", "install", ...]. This is the standard
    Windows-safe pattern and a no-op cost on POSIX.

• scripts/config.py

  • Complete USER_AGENT with the full Chrome 131 string.

How to verify

# Fresh clone, no Windows-specific env setup
cd /path/to/clone
python scripts/run.py auth_manager.py status
# Before this PR: UnicodeEncodeError on Windows
# After this PR: prints "🔐 Authentication Status: ..." correctly

# venv bootstrap from scratch (delete .venv and rerun)
rm -rf .venv
python scripts/run.py auth_manager.py status
# Before: "Failed to install dependencies" on Windows
# After: clean bootstrap, status prints

Out of scope (separate PRs if welcome)

A few smaller things I noticed while debugging but kept out of this PR to keep
review focused. Happy to file separately if any are interesting:

• auth_manager.is_authenticated() only soft-warns when state is older than
  7 days — would benefit from a hard cutoff (e.g., 14 days) so callers don't
  blunder into expired-cookie territory without a clear error.
• notebook_manager.update_notebook() exists on the class but isn't exposed
  on the CLI; would be nice to have an update subcommand.
• references/troubleshooting.md consistently says chromium but the skill
  installs Chrome (per setup_environment.py line ~76); also the Linux-only
  pkill -f chromium doesn't help Windows users.

Notes

• All three changes are minimal, self-contained, and behavior-preserving on
  non-Windows platforms (sys.stdout.reconfigure is a no-op when stdout is
  already UTF-8; python -m pip works the same as pip on POSIX; the longer
  USER_AGENT is just more accurate).
• Comments at each change site explain the why (Windows constraint), not
  the what, so future readers don't have to re-derive the FileLock or
  cp1252 reasoning.