feat: add design-md export target (DESIGN.md generator)

[nujovich]

Summary

Adds a new deterministic design-md export target that converts the resolved mint-ds.tokens.json into a human-readable DESIGN.md — a portable single source of truth for a project's design system, suitable as context for AI-assisted design workflows.

• mint-ds export --target design-md (alias design) → writes DESIGN.md
• Fully deterministic (no LLM / no network), mirroring the existing dtcg target
• Sections: Colors (per-color 50–900 scale tables), Typography (families / sizes / weights / line heights), Motion (durations / easings), Spacing, Border radius, Shadows
• Empty categories are omitted; brand falls back to # Design System; per-color description rendered only when present; table cell values escape |

Implemented as a new export target (not a new subcommand), reusing the existing resolveTarget / EXPORT_OUTPUT / cmdExport plumbing (--out, --stdout).

Closes #26.

Changes

• lib/design-md.mjs — pure convertTokensToDesignMd(tokens) converter
• lib/prompts.mjs — register the design-md target + design alias
• bin/mint-ds.mjs — deterministic cmdExport branch + --help example
• package.json — add lib/design-md.mjs to the published files
• examples/frankenstein/DESIGN.md — golden snapshot
• README.md — document the target

Test Plan

• Unit tests for the converter (brand fallback, per-color tables, description note, pipe escaping, omit-empty, determinism)
• Golden snapshot against the frankenstein fixture
• End-to-end CLI test (export --target design-md --stdout, plus design alias)
• Full suite green (534 tests)
• npm run check:pack passes (new lib file in published tarball)