Add a --validate mode to check config without executing jobs

[peopleworks]

What

Add a --validate (config-check) mode: load appsettings.json, validate every job (cron expression, timezone, command), print a report, and exit without executing anything.

Why

Lets users catch a bad cron/timezone before deploying as a service. Great for CI too.

Suggested approach

• Reuse the existing startup validation logic.
• On --validate, run validation, write a human-readable report, and return a non-zero exit code if any job is invalid.

Acceptance criteria

• --validate exits 0 when all jobs are valid, non-zero otherwise
• Clear per-job report (OK / the specific problem)
• No commands are executed in this mode
• Documented in the Quick start / CLI section

Good for newcomers — comment to claim it.