chore: remove codex output profile and mattpocock-skills remote entry

Neither has a real consumer: Codex is not a supported target, and
mattpocock-skills was the sole remote marketplace.packages[] entry
forcing apm-marketplace-check and apm-pack-check-clean to git
ls-remote on every push. Removing both drops .agents/plugins/marketplace.json
(the codex output artifact) and makes every pre-push hook resolve
fully offline. Updates README, AGENTS.md, gates.md, architecture.md,
and ADR-0015/ADR-0021 to match.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YR2CjVumUbEGWcMikcoXBD

docs/adr/0021-plugin-descriptions-state-a-domain-boundary.md

@@ -95,6 +95,10 @@ This decision covers the six plugins this repo authors. The root marketplace als
 `mattpocock-skills`, a third-party package whose description is not this repo's to write; its entry
 is out of scope and is left as published upstream.
 
+*(Note, 2026-09-13: `mattpocock-skills` has since been removed from the root marketplace. This
+section's scope statement is retained as the reasoning behind the boundary; the entry it describes
+no longer exists.)*
+
 ## Decision
 
 **A plugin's published `description` states the plugin's domain boundary. It does not enumerate the