fix(deploy): require supported Python runtime

[oosuhada]

Summary

Hardens the M8 macOS deployment path after production deployment exposed that non-interactive SSH resolves /usr/bin/python3 to Python 3.9 on the Mac mini. M8 requires Python 3.11+ because the server uses modern type syntax and datetime.UTC.

Changes

• validate Python 3.11+ before touching launchd
• prefer supported Homebrew Python on Apple Silicon/Intel
• keep PYTHON_BIN as an explicit override
• document the runtime requirement and SSH PATH pitfall

Validation

• bash -n deploy/install_macos.sh
• unsupported /usr/bin/python3 fails before launchd modification
• 51/51 Python tests pass
• deterministic experiment smoke passes
• artifact validation passes
• release audit passes

Production is currently healthy on /opt/homebrew/bin/python3 3.14; this PR makes that selection reproducible for future installs.