Adds license considerations to configurable work item types doc

Why is this change being made?

Add implementation details and license tier considerations to the configurable work item types design document, including Ruby namespace organization, downgrade behavior, and custom type limits.

Changes

• Added "Implementation Details" section documenting:
  • WorkItems::TypesFramework namespace structure for organizing type-related functionality
  • Separation between system-defined and custom type classes
• Added "License and tier considerations" section documenting:
  • Downgrade behavior: keep existing configurations but disallow mutations
  • Custom type limit of 30 per namespace/organization for Premium tier
  • Future tier differentiation approach for hierarchy and other advanced features
• Added decision registry entries:
  • Entry #6 (closed): Use of WorkItems::TypesFramework namespace
  • Entry #7 (closed): License downgrade strategy

Decision sources

• Use WorkItems::TypesFramework namespace
• Investigation: Handle license downgrades for custom work item types
• Team sync discussion (2025-11-05) on downgrade strategy
• Alignment with existing downgrade patterns for statuses and custom fields

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Commits

• Adds license considerations to configurable work item types doc

Also adds ruby namespace for type-based functionality

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