docs: expand README quickstarts, auth, and scripting guidance

[however-yir]

Summary

• add a command quickstart matrix and clarify the shortcut vs explicit command forms in README
• document auth modes, credential storage locations, JSON output patterns, and shell workflows for agent usage
• add troubleshooting for download paths and generated file names
• sync docs/cli-design.md with the current command tree and config.json / credentials.json behavior

Testing

• git diff --check
• npm test (fails locally: bun is not installed in this environment)