feat: create a demo org wide workflow template for DBT Platform

[simon106]

What

Demo of how we could implement organisation‑wide Release Promotion workflow covering Dev → Stage → Prod for DBT Platform

Why

Looking to standardise how service teams run lifecycle there applications

How this has been tested

• I have tested locally
• Testing not required

Reviewer Checklist

• I have reviewed the PR and ensured no secret values are present

[github-actions]

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