Show cache hit rate percentage

[WynnD]

Feature Request: Show cache hit rate percentage

Currently, cache read tokens are tracked but the cache hit rate isn't displayed. This would help understand how effective the prefix caching is.

Proposed Implementation

Add cache hit rate to the /savings output:

Cache Hit Rate: 71.4%
  Cached tokens: 1,234,567
  Fresh tokens: 3,456,789

Why This Is Useful

1. Diagnose cache effectiveness - Is prefix caching working as expected?
2. Spot issues - Low hit rate might indicate problems
3. Encourage longer conversations - Higher hit rate with more context

Implementation

Calculate:

cacheHitRate = cacheReadTokens / totalInputTokens × 100

Priority

Low (nice to have)