Adding a new agent: Persona Ad Gen - AI-Powered Advertising Scene Generator

[mbettan]

This pull request introduces the initial implementation of Persona Ad Gen, an intelligent agent built with the Agent Development Kit (ADK). This agent transforms user-uploaded photos into compelling, persona-driven advertising scenes.

The core functionality guides a user through a story-based brief collection process, captures key audience details, and leverages the new gemini-2.5-flash-image-preview model to generate four unique advertising scenes from a single source image. This provides a complete, end-to-end solution for AI-powered creative content generation.

1. PersonaAdGenAgent: The main agent that orchestrates the step-by-step brief collection and user interaction flow.
2. CreativeAgent: A sub-agent dedicated to generating four distinct advertising scenes based on the finalized persona and uploaded image.
3. Story-Driven Briefing: Replaces traditional forms with an interactive narrative to build the ad's context.
4. Multi-Scene Generation: Creates and saves four unique ad concepts from one base image.
5. Automatic Headline Generation: Produces compelling headlines that align with the persona's story.
6. GCS Artifact Management: Reliably saves all generated images and briefs as artifacts for easy access and download.

[mbettan]

Any updates on approval?