feat: harden workflows for day 16 using stf checks

[anmolnagpal]

Summary

• Day 16 implementation for Project 0.15 #2: terraform-digitalocean-labels.
• Switched reusable workflow call from tf-checks.yml to stf-checks.yml.
• Added explicit provider: digitalocean for STF job input.
• Reusable workflow sources remain restricted to clouddrove/github-shared-workflows only.

Ticket Mapping

• Project: anmolnagpal Project 0.15 #2
• Ticket: Day 16: Harden terraform-digitalocean-labels defaults + examples

Checklist

• Implementation branch created
• Workflow updated to STF checks
• Provider input set to digitalocean
• PR opened and linked to board ticket
• All required checks green
• No merge performed