Plugin & skill marketplace setup

Ship skills (portable instructions), plugins (runtime wiring), and marketplaces (catalogs) without mixing them up.

Three layers (normative)

Layer	What it is	Install unit	Skill Steward home
Skill	`SKILL.md` per agentskills.io	`npx skills add owner/repo --skill name`	`skills/{name}/`
Plugin	Manifest + hooks/MCP/rules/commands	Per-agent (see matrix)	`plugins/{id}/` (ADR 0004)
Marketplace	Catalog of plugins/skills	Add marketplace, then install plugin	Product repos; Guild uses public Git + skills.sh

Rule: Instructions only → skill. Event hooks or multi-file wiring → plugin (references skills by id, do not fork SKILL.md).