Add warning when pending migrations exist

Summary

Add a warning message when there are pending migrations that haven't been applied.

Current Behavior

om migrate status shows pending count but there's no proactive warning when using other commands or at startup.

Proposed Behavior

1. CLI Warning: When running om migrate status, show a colored warning if migrations are pending:

   ⚠️  Warning: 3 pending migrations not applied. Run 'om migrate run' to apply.
   

2. Optional startup warning (stretch goal): Add a function that applications can call to check migration status and log a warning if behind.

Implementation Notes

• Update status command output to highlight pending migrations
• Consider adding --check flag that exits non-zero if behind (useful for CI)

Start with the om migrate status command and inspect how it reports the pending migration count. The core work is done when pending migrations produce a colored warning with the count and suggested command; the optional startup check and --check flag are additional scope to clarify before implementation.