feat(mint): add dtcg export prompt with structured dimension token {value, unit} format

[nujovich]

Card: https://github.com/nujovich/mint-radar/issues/11

What: Adds an LLM-based dtcg export prompt to EXPORT_PROMPTS in lib/prompts.mjs. This prompt instructs the LLM to generate DTCG v1 Format Module JSON with structured dimension tokens ({value, unit} objects instead of flat strings), proper $type/$value organization, shadow arrays, and typography sub-groups. The deterministic dtcg-exporter.mjs path is unchanged — this adds a complementary LLM-driven code path for richer DTCG generation.

Why: Style Dictionary v5.4.0 adopted DTCG v2025.10 dimension tokens as {value, unit} objects. Mint's deterministic DTCG exporter already handles this format, but the LLM-based export pipeline (used for all targets via buildExportPrompt) had no dtcg prompt builder. This closes the gap so that dtcg can function as a proper export target in both deterministic and LLM modes.

Decision: Nadia chose Option B (differentiation approach — implement code plan as described).

Milestones

• Milestone 1 — Add dtcg export prompt to EXPORT_PROMPTS in lib/prompts.mjs with DTCG {value, unit} dimension token instructions
• Milestone 2 — Add DTCG type definitions to lib/types.ts for dimension, typography, and shadow tokens
• Milestone 3 — Update bin/mint-ds.mjs to support --format dtcg flag for token generation flow
• Milestone 4 — Add test fixture and tests for DTCG output validation

How to test:

npm test

[nujovich]

All milestones complete. Ready for review when you are.