docs: keep Terraform and scripts in appmod-blueprints under patterns/workshop/

Relocate TF/scripts destination from platform-engineering-on-eks internal
GitLab repo to patterns/workshop/terraform/ and patterns/workshop/scripts/
within appmod-blueprints. The internal repo contains only workshop content
and instructions, not infrastructure code.

Key changes:
- Task 1.5: 'Move TF to external repo' → 'Relocate to patterns/workshop/'
- Task 2.3: simplified dependencies (no longer depends on Epic 4)
- Epic 4: 'Move to external repo' → 'Reorganize within appmod-blueprints'
- Tasks 4.1-4.7: rewritten for internal reorganization
- Target repo structure: added patterns/workshop/terraform/ and scripts/
- All impact tables, risk mitigations, execution order updated
- DESIGN.md: 4 references updated to patterns/workshop/
- References to platform-engineering-on-eks: 87→10 (all content/instructions role)

Author: allamand