ci(reusable): restore sync-all private-repo S3 loading

[NWarila]

Summary

• Restore additive aws s3 sync loading for private repo definitions in the reusable deploy workflow.
• Remove the unused private_repos_files workflow input and update the reusable header comments/caller example for sync-all loading.
• Keep private_repos_prefix as the override point, with the existing default prefix unchanged.

Safety

• The S3 fetch step remains gated by if: ${{ !inputs.plan_only }}.
• The apply step remains gated by if: ${{ inputs.apply && !inputs.plan_only }}.
• The sync command is additive and does not use --delete.
• Only .github/workflows/reusable-terraform-deploy.yaml changed.

Validation

• git diff --check origin/main..HEAD
• signed commit verification via git log -1 --show-signature --format=fuller
• actionlint was not available locally; GitHub Actions should run the workflow validation checks.