full-day workshop: Add Azure prerequisites, cost warning, and resource cleanup

[softchris]

Problem

The full-day README lists only a GitHub account and Copilot access as required resources, but Exercise 8 requires an Azure subscription, permission to create resources and identities, and GitHub permissions needed by azd pipeline config. The exercise provisions billable resources and ends without an azd down or equivalent cleanup step.

An independent learner can complete most of the day before discovering they cannot deploy, or leave paid resources running after the workshop.

Suggested change

Move deployment prerequisites to the workshop entry page and add an explicit teardown section after deployment verification.

Acceptance criteria

• The README states the Azure subscription and permission requirements before learners begin.
• Exercise 8 includes a preflight check for Azure and GitHub authentication/permissions.
• The workshop identifies that deployed resources may incur charges.
• A tested cleanup command such as azd down --purge is provided with an explanation of what it removes.
• Learners verify the resource group and related credentials are removed.