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/agent/src/agent.ts

@@ -335,6 +335,7 @@ export class Agent {
 					output: 0,
 					cacheRead: 0,
 					cacheWrite: 0,
+					totalTokens: 0,
 					cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
 				},
 				stopReason: this.abortController?.signal.aborted ? "aborted" : "error",