fix: show update notice on next run

[ZeeshanAdilButt]

Summary

• The previous approach used Promise.race with a 500ms window after the command completed. For fast commands (help, version, status) the window often expired before the npm registry responded, so users never saw the update notice.
• Switched to a write-on-check, show-on-next-run pattern used by npm, Homebrew, and others:
  • showPendingUpdateNotice() — reads a notice file at process start (before command output), prints the banner, deletes the file
  • scheduleUpdateCheck() — fires a background registry check with no await; if a newer version is found, writes the notice file for the next run
• Notice now appears before command output (top of run) rather than appended at the bottom
• Existing maybeNotifyUpdate export kept for backward compat with any direct imports

Test plan

• npm test — all 204 tests pass
• Run any fast command twice after an old version is in cache: notice appears on the second run
• OLOSTEP_NO_UPDATE_CHECK=1 olostep version — no notice written or shown
• olostep update — no notice written or shown

[ZeeshanAdilButt]

consolidating into single PR