Auto-update OpenAdapt on launch

[hechibing]

Refs #755

Bounty: $100

Changes:

• Add CLI auto-update on launch: checks PyPI for a newer openadapt version, upgrades via pip, then re-execs the CLI
• Add rate limiting (once per 24h) via ~/.openadapt/autoupdate.json
• Add OPENADAPT_DISABLE_AUTO_UPDATE=1 escape hatch and skip in CI
• Make CLI and package version report the installed distribution version (no more hard-coded 1.0.0)

Verification:

• python -m py_compile openadapt/init.py openadapt/version.py openadapt/autoupdate.py openadapt/cli.py