Add deterministic pixel robot avatars for agents

[0xroylee]

Summary

• Add a reusable 90s-style pixel robot avatar component with deterministic seed-based variants
• Keep generation logic pure and tested in shared utility and type modules
• Swap the Agents table row icon for the new avatar component
• Ignore generated .superpowers/ workspace artifacts

Testing

• Added unit coverage for stable recipe generation, blank-seed fallback, and avatar size normalization
• Local typecheck, build, browser verification on /agents, and full repo test suite all passed