Add prototype skill and related documentation for interactive design exploration

skills/in-progress/README.md

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+# In Progress
+
+Skills that are still being developed. They're not ready to ship — expect rough edges, breaking changes, and abandoned experiments. They're excluded from the plugin and the top-level README until they graduate to a stable bucket.
+
+- **[writing-beats](./writing-beats/SKILL.md)** — Shape an article as a journey of beats, choose-your-own-adventure style. Pick a starting beat, write only that beat, then pivot to the next, until the article reaches a natural end.
+- **[writing-fragments](./writing-fragments/SKILL.md)** — Grilling session that mines you for fragments — heterogeneous nuggets of writing — and appends them to a single document as raw material for a future article.
+- **[writing-shape](./writing-shape/SKILL.md)** — Take a markdown file of raw material and shape it into an article paragraph by paragraph, arguing format choices at each step.