add opt-in line numbers and tighten docs

[hyle]

Summary

Add opt-in line numbers for review-oriented exports and tighten the contributor docs around fuori's design boundaries.

Changes

• Add --line-numbers to prefix exported code lines inside fenced blocks
• Include Line numbers: on in the export header when the flag is enabled
• Keep line numbering as a render-only concern so metrics and output stay aligned
• Update README options, examples, and output-format documentation for numbered exports
• Rewrite docs/design.md into a shorter design-boundary document focused on scope, non-goals, artifact shape, and render/selection philosophy
• Clean up a small main() consistency issue so recursive ignore-load failures go through cleanup
• Fix a README typo in the Git-selection semantics section

Why

Line numbers are useful for code review and discussion, but they add noise for pure context-packing, so they remain opt-in. The header note makes numbered artifacts self-describing, and the design-doc rewrite keeps contributor guidance concise and durable instead of duplicating README behavior.

Verification

• Ran make test successfully