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

scripts/sync-marketplace-mirror.sh

@@ -6,11 +6,10 @@ set -euo pipefail
 # that same file at .claude-plugin/marketplace.json directly, but also has a legacy
 # convention path at .github/plugin/marketplace.json (see
 # plugins/kyberforge/docs/research/docs/github-copilot-plugins/marketplace.md) -- and
-# that path is a mirror of the Claude output, not a separate apm
-# output profile (apm only ships "claude" and "codex" mappers; codex writes a
-# differently-shaped file to .agents/plugins/marketplace.json, not this path). This
-# script keeps that legacy mirror byte-identical to .claude-plugin/marketplace.json
-# instead of letting it silently drift (see issue #90 comment thread).
+# that path is a mirror of the Claude output, not a separate apm output profile (this
+# repo declares only the "claude" output). This script keeps that legacy mirror
+# byte-identical to .claude-plugin/marketplace.json instead of letting it silently
+# drift (see issue #90 comment thread).
 
 # Hard error, not a `|| pwd` fallback. Every path this script touches hangs off
 # REPO_ROOT, and both of its exits-0 paths are "the files agree" or "neither file