fix(coding-agent): accept sparse leap-day schedules

[chai1any]

Summary

• search a complete Gregorian calendar cycle when resolving the next five-field cron run
• skip non-matching calendar days so sparse schedules remain fast without a minute-by-minute multi-year scan
• add a timezone-independent regression for a leap-day schedule created in 2025

Problem

nextCronRunAfter() currently stops after 366 days. A valid schedule such as 0 0 29 2 * is therefore rejected during creation in most years because its next occurrence can be several years away:

Cron schedule did not match within one year: 0 0 29 2 *

Gregorian calendar dates repeat every 400 years, which provides a finite correctness bound. Skipping calendar days that cannot match keeps that bound practical and also rejects impossible dates deterministically.

Verification

• npx tsx ../../node_modules/vitest/dist/cli.js --run test/cron-jobs.test.ts (54 tests passed)
• npm run check
• cron suite also passed under UTC, America/New_York, Europe/Berlin, Pacific/Apia, and Australia/Lord_Howe
• manual reproduction now resolves 0 0 29 2 * from March 2025 to February 29, 2028

Note

Fix nextCronRunAfter to accept leap-day cron schedules beyond one year

• Replaces the one-year minute-by-minute scan in nextCronRunAfter with a 400-year Gregorian window, allowing sparse schedules like 0 0 29 2 * (leap day) to resolve correctly.
• Adds a calendar-day prefilter via a new matchesCronCalendarDay helper that skips non-matching days in whole-day increments instead of iterating minute-by-minute.
• Impossible dates (e.g. 0 0 31 2 *) now throw 'Cron schedule has no matching date: ...' instead of 'Cron schedule did not match within one year: ...'.
• Behavioral Change: the scan window is now 400 years instead of 1; schedules that previously errored after one year may now resolve.

^{Macroscope summarized d53b62e.}