docs: add CHANGELOG.md documenting fork differences

[CybotTM]

Summary

• Add comprehensive CHANGELOG.md following Keep a Changelog format
• Document all features, fixes, and changes since fork from robfig/cron
• Include migration guide and differences comparison table

Content

• Added: Input validation, timezone validation, step range validation, timeout wrapper, slog adapter, EntryID uint64, DST handling, multi-platform CI, benchmark suite
• Fixed: Upstream issues SkipIfStillRunning Chain Not Applied When Manually Calling Entry.Job.Run() robfig/cron#551, cron.ParseStandard("TZ=0") causes panic robfig/cron#554, Fix panic when missing optional fields or descriptor after timezone robfig/cron#555, NewParser panic, time backwards handling
• Changed: Go version, module path, CI/CD, linting
• Security: Timezone validation, input limits, security scanning tools

Test plan

• Documentation only change
• Links are valid
• CI passes

Closes #22