Merchant-state safety: settings_data.json doctrine and sold-out grid state

Problem: Spark's docs treat configs/settings_data.json as the defaults file but never warn that pushing it overwrites live merchant Theme Editor state, and a plain ntk push uploads it. Separately, sold-out products vanish from product grids entirely, so a curated grid can silently render fewer cards than configured with no explanation to the shopper or the merchant.

Evidence: docs/extending-spark.md:18-30 (no overwrite warning); partials/product_card.html:4 wraps the whole card in an availability check; docs/onboarding-claude-code-prompt.md exists but is untracked.

Done when:

• Spark docs carry the same settings-safety contract as the next-theme-dev skill: never push settings_data.json by default; when merging settings, use the live file as the base.
• Product cards render a sold-out state (badge or greyed card, configurable) instead of disappearing.
• The onboarding prompt doc is committed or deliberately dropped.

Start with docs/extending-spark.md:18-30 and compare its settings guidance with the next-theme-dev skill contract. Inspect partials/product_card.html:4 to understand the current availability check, then decide how the sold-out state should be represented and how docs/onboarding-claude-code-prompt.md should be handled. Done means safe settings guidance, visible sold-out cards, and a deliberate outcome for the onboarding prompt.