Add update notification (like Claude Code)

[lionuncle]

Summary

• Checks PyPI for newer versions and shows a boxed notification at the end of each run when an update is available:

  ╭─────────────────────────────────╮
  │ Update available: 0.1.1 → 0.2.0 │
  │ pip install -U rpr              │
  ╰─────────────────────────────────╯
  

• Runs in a background thread — zero impact on review speed. The check happens in parallel with the actual review work.
• Results are cached for 1 hour at ~/.cache/rpr/update-check.json so most runs don't hit the network at all.
• All failures (network errors, cache corruption, PyPI downtime) are silently ignored — the notification simply doesn't appear.

Test plan

• Run rpr <pr> --dry-run — verify no notification when on latest version
• Temporarily set __version__ to 0.0.1 and run — verify notification appears
• Verify ~/.cache/rpr/update-check.json is created after first run
• Run again within 1 hour — verify no network call (check cache timestamp)
• Disconnect network and run — verify no errors, notification just doesn't show

🤖 Generated with Claude Code