Docs/windows instructions

[hanley-development]

Summary

• Document Windows as a supported runtime path now that Codex home resolution is cross-platform.
• Add PowerShell examples for running the script, local-only usage, syntax checks, and optional CODEX_HOME setup.
• Replace hardcoded ~/.codex README assumptions with resolved <Codex home> wording for auth, local usage, troubleshooting, and privacy guidance.

Why

The code now resolves Codex data from CODEX_HOME when set, otherwise Path.home() / ".codex". The README still described macOS/Linux-only usage and literal ~/.codex assumptions, which made the workflow unclear for Windows users and custom Codex home layouts.

User Impact

• Existing macOS/Linux users keep the same default behavior and commands.
• Windows users get direct PowerShell commands using the Python launcher.
• Users with custom Codex data locations know to set CODEX_HOME before running reports.

Test Plan

• python -B -m unittest
• python -m py_compile .\codex_usage.py
• ruff check .\codex_usage.py .\tests
• ruff format --check .\codex_usage.py .\tests
• pyflakes .\codex_usage.py .\tests