Rewrite documentation to match codebase

[jsk11235]

Summary

• Rewrote all documentation based solely on reading the code (not existing docs)
• README.md now accurately covers installation, CLI, configuration (providers, frameworks, groups), env var expansion, exit codes, and library usage
• Added ARCHITECTURE.md with detailed technical design (module structure, core abstractions, execution flow, concurrency model)
• Simplified TESTING.md to focus on running tests and required checks
• Simplified CLAUDE.md for AI agent guidance with key files and patterns
• Removed outdated files: AGENTS.md, STYLE_GUIDE.md, .claude/agents/*.md
• Updated code doc comments in lib.rs, config.rs, provider.rs to include ModalProvider

Test plan

• cargo fmt --check passes
• cargo clippy passes
• Documentation accurately reflects actual code

🤖 Generated with Claude Code

[github-actions]

Vet found 0 issues.

[jsk11235]

Closed in favor of newer docs update