feat: surface OpenAI image-gen usage in image_openai_agent

Part of #1415. ## What OpenAI's `images.generate` (gpt-image-1) returns `response.usage` with `input_tokens` / `output_tokens` / `total_tokens`. Currently `src/agents/image_openai_agent.ts` throws it away and returns `{ buffer }`. Update the agent to return: ```ts return { buffer, usage: { provider: "openai", model: usedModel, inputTokens: response.usage?.input_tokens, outputTokens: response.usage?.output_tokens, totalTokens: response.usage?.total_tokens, }, }; ``` Wire the GraphAI `registerCallback` in `src/actions/images.ts` (`images_graph_data`) to push `result.usage` into `context.usageCollector` (depends on #1). ## Acceptance - [ ] Agent return shape extended (backward compatible — existing consumers only read `buffer`) - [ ] Callback registered in `src/actions/images.ts` - [ ] Unit test or integration test asserting collector receives a record after generating one image (mock the agent or use `media_mock_agent`) - [ ] Cache hits NOT recorded (verify via test) ## Dependencies - #1 (UsageCollector)