fix(ci): remove secrets from if expressions in preview-cron

[dante01yoon]

Summary

• Bug: secrets.* in if: expressions causes GitHub Actions to reject the entire workflow file (Unrecognized named-value: secrets). This broke all scheduled preview-cron runs since PR Fix preview cron: read VERCEL_PREVIEW_ALIAS from secrets #743 (March 27).
• Fix: Move secret-existence checks from if: conditions into shell scripts.
• Refactor: Merge two duplicate alias steps into one parametric step that selects the correct alias via API_ENV.

Root Cause

# ❌ GitHub Actions does not allow secrets in if expressions
if: matrix.is_main && matrix.api_env == 'production' && secrets.VERCEL_PREVIEW_ALIAS

# ✅ Check in shell instead
if: matrix.is_main
run: |
  if [ "$API_ENV" = "production" ]; then ALIAS="$PROD_ALIAS"; else ALIAS="$TEST_ALIAS"; fi
  if [ -n "$ALIAS" ]; then npx vercel alias ...; fi

Environment Matrix (unchanged)

Scenario	ref	api_env	Alias domain
Preview Prod	main	production	VERCEL_PREVIEW_ALIAS
Preview Test	main	test	VERCEL_PREVIEW_TEST_ALIAS
PR Preview	PR branch	test	(no alias, comments URL on PR)

Test Plan

• workflow_dispatch trigger succeeds (currently fails with parse error)
• Scheduled runs resume every 15 minutes
• Production API preview aliased to VERCEL_PREVIEW_ALIAS
• Test API preview aliased to VERCEL_PREVIEW_TEST_ALIAS
• PR previews still post comment with URL