docs: add CLAUDE.md and deployment architecture documentation

[dengwx2026]

Summary

Add project-level documentation to improve developer onboarding and codebase understanding:

1. CLAUDE.md — Project overview, architecture, code conventions, testing guide, git workflow
2. docs/dev/deployment/README.md — Deployment architecture documentation with Mermaid diagrams, Docker usage analysis, and proposals for need_docker test marker and early Docker validation in SandboxManager

Motivation

• New contributors need a quick reference for project structure, conventions, and workflow
• The deployment architecture (Operator, DeploymentManager, Deployment hierarchy) lacks documentation
• test_get_status and similar Docker-dependent tests fail unclearly when Docker is unavailable — documented proposals for improvement