Define plugin distribution domain terms

[ericjuta]

Summary

• define canonical skill, plugin source, host artifact, repo marketplace, and universal directory
• keep repo marketplace language distinct from OpenAI public submission
• provide the shared vocabulary for Wayfinder map Wayfinder: Codex repository marketplace distribution #19

Verification

• bun run fmt:check

Wayfinder: #19

[ericjuta]

Closing without merge: this documentation branch was created during Wayfinder charting but was not part of the authorized route. Map #19 and decision #22 remain authoritative.