Add DELETE Endpoint for Cleaning Up API-Generated Presentations

Add a DELETE /v1.0/gammas/{gammaId} endpoint to allow programmatic cleanup of API-generated presentations. Use Case: After several months of operation, my "API generated" workspace tab has accumulated hundreds of presentations that serve no purpose. I have no way to clean these up programmatically, forcing manual deletion through the web UI. Proposed API: DELETE /v1.0/gammas/{gammaId} X-API-KEY: sk-gamma-xxxxxxxx Response 200: { "deleted": true, "gammaId": "g_abc123" } Response 404: { "message": "Gamma not found", "statusCode": 404 } Optional enhancement: DELETE /v1.0/gammas with a batch payload for bulk cleanup.