Position CLI for AI agent billing

[sionsmith]

Summary

• update README headline and intro around AI agent time tracking, AI agency billing, and AI-native service companies
• add SEO-aligned links into the Keito AI-native service billing solution cluster
• align Cargo package description with the new positioning

Validation

• cargo metadata --no-deps --format-version 1
• cargo fmt --check
• cargo test --all-targets

GitHub repo description has also been updated.