feat: Consolidate Stable Diffusion agent (gaia sd) into Agent UI

[kovtcharov-amd]

Part of #759.

Current State

The Stable Diffusion agent lives at src/gaia/agents/sd/ and is accessible via gaia sd CLI only. It supports multiple model variants (SD-1.5, SD-Turbo, SDXL) but has no UI — image generation is text-in / file-out via the terminal.

Goal

Integrate the SD agent into the main Agent UI (src/gaia/apps/webui/) so users can generate, preview, and iterate on images without leaving the app.

What Needs to Happen

1. Add SD agent to Agent UI's agent routing/selection
2. Build an image generation UI panel in Agent UI (prompt input, model selector, generated image display)
3. Stream generated images back to the UI (base64 or file URL)
4. Keep gaia sd CLI intact (no deprecation needed — CLI is the thin wrapper, not a standalone app)

References

• src/gaia/agents/sd/ — agent implementation
• src/gaia/apps/webui/ — target Agent UI