docs: streamline agent instructions and design documentation

[gricha]

Summary

This PR streamlines Perry's core documentation to improve clarity and reduce redundancy:

• AGENTS.md: Refactored from 159 to 100 lines with clearer structure and validation requirements
• DESIGN.md: Updated from vision document to current implementation status with concrete architecture details

Key Improvements

AGENTS.md Changes

• Added architecture reference pointing to DESIGN.md for comprehensive specs
• Simplified validation workflow with clear manual testing requirements
• Streamlined task management and testing guidelines
• Removed redundant sections while preserving critical implementation patterns

DESIGN.md Changes

• Added implementation status checklist showing current vs planned features
• Clarified component descriptions and data models
• Improved API specifications with concrete oRPC examples
• Better session management and workspace lifecycle documentation

Impact

• For developers: Clearer onboarding and validation processes
• For contributors: Better understanding of current architecture vs future plans
• For maintenance: Reduced documentation drift between files

The documentation now provides a clearer path from high-level design (DESIGN.md) to implementation guidelines (AGENTS.md) while eliminating redundant information.