Why: plugin skills were written to the skill root unvalidated and the
bundled recipes carried no name — both now go through the library's
spec validator (name required + kebab-case + equals dir, description
required, known-keys whitelist).
Evidence: go test -count=1 ./... green; TestV2InvalidSkillRejected pins
the fail-closed path with plugin context + field location; Seed
validates each SKILL.md before write (recipes dogfood-compliant);
nested pins carry agent-native#28 (the validator).
Impact: invalid plugin skills now abort the plugin load with a located
error instead of landing silently; the architecture baseline gains
backend/plugins→backend/skills.
Refs: Agent-Native#28, Agent-Squad#32, N2 plan WP4
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