Update README and add comprehensive documentation guides

README improvements:
- Add Quick Start section with direct usage and alias options
- Add "Your First Script" section with working example
- Document environment variables (TOME_ROOT, TOME_EXECUTABLE, etc.)
- Add Troubleshooting section for common issues
- Reorganize Features section and add Table of Contents

New docs:
- docs/writing-scripts.md: Guide for writing scripts in multiple languages,
  explains USAGE vs SUMMARY (both work identically), environment variables,
  organization patterns, and best practices
- docs/completion-guide.md: How to implement script-level completions with
  examples in bash, python, typescript, ruby, and advanced patterns
- docs/migration.md: Migration guide from original tome/sub with checklist
  and troubleshooting

Clarifies that USAGE and SUMMARY are interchangeable keywords (both
supported for backward compatibility). Adds practical examples based on
storage-cli usage patterns.

Author: zph