Add --validate config-check mode

[uttam12331]

Closes #3.

Adds a --validate (alias --check) mode that loads appsettings.json, validates every job in ScheduledCommands (required Id/Command, a parseable CronExpression via Cronos, and a resolvable TimeZone via the existing TimeZoneHelper), prints a per-job report, and executes nothing. It exits 0 when all jobs are valid and non-zero otherwise — useful as a pre-deploy or CI gate.

• New ConfigValidator reuses the existing ScheduledCommand model and TimeZoneHelper checks.
• Program.Main short-circuits to validation before building the host, so no commands run.
• Documented in the README Quick start.

Verified locally: dotnet build clean; valid config -> exit 0, broken cron / missing command -> exit 1 with a clear per-job report; confirmed no commands execute.