fix(pm): rewrite auto-update to fix never-triggering bug

The old auto-update used in-memory global state (AtomicBool) that reset
on every process start, so updates could never actually trigger. The
background version check was also killed by runtime shutdown before
completing.

New design:
- Read cached version from ~/.utoo/remote-version.json on startup
- If cache is fresh and version differs, try update immediately
- If cache is missing/expired, background fetch + update if needed
- 24h cooldown after failed attempts to avoid nagging
- UTOO_FORCE_UPDATE=1|true to bypass dev/CI guards for testing
- Suppress install output during auto-update, show colored status

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: elrrrrrrr