Add marketplace publishing design documentation

Co-authored-by: hotlong <50353452+hotlong@users.noreply.github.com>

Author: Copilot

docs/design/marketplace-publishing.md

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+# Design Document: Marketplace Protocol — Package Publishing & Distribution
+
+> **Author:** ObjectStack Core Team  
+> **Created:** 2026-02-17  
+> **Status:** Design Specification  
+> **Target Version:** v3.2 – v4.0
+
+This document defines the **Marketplace Protocol** for ObjectStack — covering how metadata-driven packages are published to the marketplace and how customers discover, install, and manage them.
+
+## Key Schema Files
+
+1. **`packages/spec/src/kernel/manifest.zod.ts`** — Package manifest schema
+2. **`packages/spec/src/kernel/package-registry.zod.ts`** — Runtime lifecycle management
+3. **`packages/spec/src/cloud/marketplace.zod.ts`** — Marketplace ecosystem schemas
+
+See full documentation in this file.