[codex] Align system Python installation

[greenthree]

What changed

• align the main README, compatibility README, Agent Skill, and Release guidance around one Node.js 18+ / Python 3.10+ installation path
• install pinned dependencies into the selected system Python user site with explicit consent and PEP 668 compatibility
• isolate pip from PYTHONHOME, PYTHONPATH, and PYTHONSTARTUP pollution
• exercise the real npm compatibility command, Windows PowerShell shim, Windows Store Python, and Ubuntu distro Python
• initialize empty workspace lock files only after acquiring the OS lock

Why

The documented system-Python flow previously failed on Ubuntu's externally managed Python and could modify a developer's host Python during clean-install tests. Concurrent first-use generation could also race while initializing an empty lock file on Windows.

User impact

Users can follow the same commands on Windows and Ubuntu without creating a virtual environment. Ubuntu dependencies stay in the user site, missing python3-pip gets an actionable error, and concurrent generation startup no longer intermittently fails.

Validation

• npm run check: 545 passed, 23 skipped
• python scripts/check_clean_install.py --json: full delivery and rebuild-equivalence passed
• npm run pack:check: 213 / 5 files
• tests.test_npm_packages: 15 passed
• concurrent generation test: 30 repeated runs passed on Windows
• WSL lock/generation regression: 3 passed
• Skill validation and git diff --check passed
• local phtest nine-problem lint passed; worktree remained clean