The diff command added unnecessary complexity and duplicated functionality
already available through the show command. Users can now use:
- `openspec show <change>` for structured change viewing
- `openspec show <change> --json --deltas-only` for delta-only views
- Standard git diff or other tools for file comparisons
This change:
- Removes ~227 lines of code and the jest-diff dependency
- Simplifies the CLI interface
- Reduces maintenance burden
- Aligns with verb-first command structure