Add repository description caching

[justinabrahms]

Summary

Implements application-level caching for repository descriptions to avoid redundant GitHub API calls.

Changes

• Added GetRepository() method: Fetches individual repositories by owner/repo name
• Added repoCache map: In-memory application-level cache for repository data
• Auto-caching: All repositories fetched from GetStarredRepos(), GetStarredReposByUsername(), GetOwnedRepos(), and GetOwnedReposByUsername() are automatically cached
• Cache management methods: CacheRepository() for pre-populating and ClearRepoCache() for clearing the cache
• Comprehensive tests: Added 5 new test cases covering caching behavior

Benefits

1. Reduced API calls: When we need to fetch a repository we've already seen (from starred/owned lists), we use the cached version instead of making another API call
2. Lower rate limit usage: Fewer API calls mean we stay well under GitHub's rate limits
3. Faster operations: Cached data returns immediately without network latency
4. Future-proof: Enables future features that might need individual repo lookups (e.g., enriching event repositories with descriptions)

Test Results

All tests pass - github package tests run in 0.214s with all 27 tests passing.