test(cron): add focused tests for parseCronExpression

[Jah-yee]

Summary

Adds focused unit tests for src/utils/cron.ts covering the cases suggested in issue #2:

• exact minute matching — verifies 30 9 * * 1-5 parses correctly
• wildcard fields — verifies * * * * * expands to full ranges
• step expressions — verifies */15 yields [0, 15, 30, 45]
• range expressions — verifies 9-17 and 1-5 ranges
• list expressions — verifies 8,12,18 hour list
• invalid field count — verifies error on 4-field expression
• empty expression — verifies error on empty string

All 7 tests pass with bun test.

Closes #2