Add prompt cache statistics tracking to /stats command

- Extend Usage struct with cache_creation_tokens and cache_read_tokens fields
- Parse Anthropic cache_creation_input_tokens and cache_read_input_tokens
- Parse OpenAI prompt_tokens_details.cached_tokens for automatic prefix caching
- Add CacheStats struct to Agent for cumulative tracking across API calls
- Add "Prompt Cache Statistics" section to /stats output showing:
  - API call count and cache hit count
  - Hit rate percentage
  - Total input tokens and cache read/creation tokens
  - Cache efficiency (% of input served from cache)
- Update all provider implementations and test files

crates/g3-core/tests/test_token_counting.rs

@@ -38,6 +38,8 @@ fn test_update_usage_only_affects_cumulative() {
         prompt_tokens: 100,
         completion_tokens: 50,
         total_tokens: 150,
+        cache_creation_tokens: 0,
+        cache_read_tokens: 0,
     };
     window.update_usage_from_response(&usage);
 
@@ -52,6 +54,8 @@ fn test_update_usage_only_affects_cumulative() {
         prompt_tokens: 200,
         completion_tokens: 75,
         total_tokens: 275,
+        cache_creation_tokens: 0,
+        cache_read_tokens: 0,
     };
     window.update_usage_from_response(&usage2);
 
@@ -156,6 +160,8 @@ fn test_cumulative_vs_used_independence() {
         prompt_tokens: 500,
         completion_tokens: 200,
         total_tokens: 700,
+        cache_creation_tokens: 0,
+        cache_read_tokens: 0,
     };
     window.update_usage_from_response(&usage);