Add prompt template management

[jwilger]

Priority: MEDIUM (Post-MVP)
Size: Large
Description: Create system for managing and versioning prompt templates. This enables prompt reuse and standardization across applications.

Context: Prompt engineering is critical for LLM performance. Centralized template management enables sharing best practices and version control.

Acceptance Criteria:

• Template storage in EventCore:
  • Template content
  • Variables definition
  • Metadata (author, purpose)
• Template features:
  • Variable substitution {{variable}}
  • Conditional sections
  • Template inheritance
• Versioning:
  • Version history
  • Diff viewing
  • Rollback capability
• Access control:
  • Read/write permissions
  • Approval workflow
  • Audit trail
• Template testing:
  • Preview with sample data
  • Performance comparison
• API for template usage
• UI for template management

Dependencies:

• Create GitHub Milestones for Development Phases #8 (EventCore for storage)
• Implement SQLx database layer #31 (UI framework)