feat: comprehensive token usage tracking for V2

Add provider-specific usage details, fix streaming usage, and return
usage from all high-level APIs (Chat.Send, Generate[T], Agent.Run).

Breaking changes:
- Chat.Send/SendMessage/SendWithImages now return (string, *Usage, error)
- Generate[T]/GenerateWith[T] now return (T, *Usage, error)
- Agent.Run/RunMessages now return (string, *Usage, error)

New features:
- Usage.Details map for provider-specific token breakdowns
  (reasoning, cached, audio, thoughts tokens)
- OpenAI streaming now captures usage via StreamOptions.IncludeUsage
- Google streaming now captures UsageMetadata from final chunk
- UsageTracker.Details() for accumulated detail totals
- ModelPricing and PricingRegistry for cost computation

Closes #2

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

v2/agent/example_test.go

@@ -25,7 +25,7 @@ func Example_researcher() {
 		agent.WithRequestOptions(llm.WithTemperature(0.3)),
 	)
 
-	result, err := researcher.Run(context.Background(), "What are the latest developments in Go generics?")
+	result, _, err := researcher.Run(context.Background(), "What are the latest developments in Go generics?")
 	if err != nil {
 		fmt.Println("Error:", err)
 		return
@@ -50,7 +50,7 @@ func Example_coder() {
 		)),
 	)
 
-	result, err := coder.Run(context.Background(),
+	result, _, err := coder.Run(context.Background(),
 		"Create a Go program that prints the first 10 Fibonacci numbers. Save it and run it.")
 	if err != nil {
 		fmt.Println("Error:", err)
@@ -97,7 +97,7 @@ func Example_orchestrator() {
 		)),
 	)
 
-	result, err := orchestrator.Run(context.Background(),
+	result, _, err := orchestrator.Run(context.Background(),
 		"Research how to implement a binary search tree in Go, then create one with insert and search operations.")
 	if err != nil {
 		fmt.Println("Error:", err)