fix(cli): validate scheduled post timestamps strictly

[WNZhao]

Summary

• reject timezone-ambiguous values passed to promote social post --schedule
• reject calendar rollovers such as February 30 and out-of-range times
• keep accepting ISO timestamps with Z or an explicit UTC offset

This prevents scheduled posts from running at different instants depending on the machine timezone.

Tests

• pnpm --filter @profullstack/sh1pt build
• pnpm exec vitest run packages/cli/src/commands/promote.test.ts (33 passed)

[WNZhao]

@ralyodio This is the PR attached to my pending uGig bug-fix bounty submission. All six required checks are green and the branch is currently mergeable. When convenient, please review it for bounty acceptance; the additional independent fixes are linked in our uGig conversation.