[WIP] Add per-repo query cache with invalidation on issue updates

[Copilot]

Implementation Plan: Per-Repo Query Cache with Issue Update Invalidation

• Explore repository structure and understand codebase
• Run existing tests to establish baseline
• Create cache package in internal/cache
  • Implement thread-safe cache structure with per-service storage
  • Add cache entry with timestamp and data
  • Add methods: Get, Set, Invalidate, InvalidateAll
• Integrate cache into agent package
  • Update listAgents to use cache
  • Update describeAgent to use cache
  • Invalidate cache on registerAgent
  • Invalidate cache on deregisterAgent
• Add comprehensive tests for cache
  • Test cache get/set operations
  • Test cache invalidation
  • Test thread safety
• Add integration tests for agent operations with cache
• Run all tests to verify functionality
• Build and verify no compilation errors

Original prompt

Create a per repo query cache, which must be invalid with any issue updates.

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