Add debugger integration and update documentation

- Add ,bt command to show backtrace from last error
- Capture SBCL backtraces when evaluation errors occur
- Show "Use ,bt for backtrace" hint after errors
- Update README with new features: syntax highlighting,
  paren matching, debugging commands, ICL_BACKGROUND env var

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>