Add -n 0 flag to disable xdist parallel execution which was causing
test collection failures. Also fix successful run count calculation.

- Use PYTEST_DISABLE_PLUGIN_AUTOLOAD environment variable (recommended in CLAUDE.md)
- Use -p no:xdist flag to explicitly disable the plugin
- Add --no-cov to avoid coverage plugin issues
- Add test-stdio-simple.ps1 to try different methods of disabling xdist

- Add setup-environment.ps1 to configure Git for Windows tee in PATH
- Simplify all scripts to use working -o addopts="" approach
- Add tee availability checks to all scripts
- Update README with clear setup instructions and troubleshooting
- Remove complex xdist workarounds in favor of simple override

Based on deep analysis, the flakiness likely occurs due to:
1. Job Object handle race conditions when tests run in parallel
2. Windows handle inheritance between test processes
3. Timing sensitivity in the minimal test that just opens/closes immediately
4. Python 3.11/3.12 specific subprocess handling changes

These scripts help reproduce the CI environment conditions:
- test-stdio-parallel-flakiness.ps1: Runs with xdist parallel workers
- test-stdio-stress-race.ps1: Rapidly creates processes to expose races

This script collects detailed information about:
- Windows version and build
- Python version and subprocess configuration
- Job Objects support
- Security software
- Process creation timing
- Handle inheritance settings

This will help identify differences between local dev environments
(where the test passes) and CI environments (where it flakes).