Add totalTokens field to Usage type

- Added totalTokens field to Usage interface in pi-ai
- Anthropic: computed as input + output + cacheRead + cacheWrite
- OpenAI/Google: uses native total_tokens/totalTokenCount
- Fixed openai-completions to compute totalTokens when reasoning tokens present
- Updated calculateContextTokens() to use totalTokens field
- Added comprehensive test covering 13 providers

fixes #130

packages/coding-agent/src/compaction.ts

@@ -32,9 +32,10 @@ export const DEFAULT_COMPACTION_SETTINGS: CompactionSettings = {
 
 /**
  * Calculate total context tokens from usage.
+ * Uses the native totalTokens field when available, falls back to computing from components.
  */
 export function calculateContextTokens(usage: Usage): number {
-	return usage.input + usage.output + usage.cacheRead + usage.cacheWrite;
+	return usage.totalTokens || usage.input + usage.output + usage.cacheRead + usage.cacheWrite;
 }
 
 /**