feat: Add CLAUDE.md for project architecture and development guidance

[moming2k]

Adds a comprehensive CLAUDE.md file to guide Claude Code (claude.ai/code) when working with this repository. This file establishes clear development patterns and architectural understanding for
AI-assisted development.

What's Added

• Architecture Overview: Full-stack structure with Next.js frontend and FastAPI backend
• Development Commands: Essential commands for local development, testing, and database operations
• Project Structure: Clear breakdown of monorepo organization and key directories
• Configuration Guide: Environment setup and port management
• Development Workflow: AI agent integration and code generation flow

[mojosolo]

Thanks for the contribution @moming2k! However, this repository already uses AGENTS.md as the canonical documentation file for all AI coding agents (see commit ef5d4ac).

Current architecture:

• AGENTS.md (13,979 bytes) - Comprehensive canonical documentation
• CLAUDE.md (595 bytes) - Lightweight pointer to AGENTS.md

The existing AGENTS.md already covers:

• Multi-agent CLI system architecture
• Development commands and workflows
• Runtime behavior and configuration
• Project scaffolding details
• Model selection mappings
• Tool normalization

Why AGENTS.md instead of CLAUDE.md:
Multiple CLI adapters auto-create AGENTS.md from apps/api/app/prompt/system-prompt.md - it's part of the project scaffolding system. Both Codex and Cursor adapters expect AGENTS.md.

Recommendation: Close this PR as the documentation architecture is already established. If you'd like to enhance the existing documentation, consider opening a PR to improve AGENTS.md instead.

See also: CONTRIBUTING.md for documentation standards.