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

.github/plugin/marketplace.json

@@ -43,19 +43,6 @@
       "category": "Productivity",
       "source": "./plugins/core"
     },
-    {
-      "name": "mattpocock-skills",
-      "description": "Skills for Real Engineers — planning, TDD, architecture, and debugging workflows from Matt Pocock's .claude directory.",
-      "version": "1.2.3",
-      "category": "Productivity",
-      "source": {
-        "source": "github",
-        "repo": "mattpocock/skills",
-        "ref": "v1.2.3",
-        "sha": "835450ef244ab7335f75d95b83e7d979eae22a6d",
-        "tag_pattern": "v{version}"
-      }
-    },
     {
       "name": "lint",
       "description": "Skills and agents for configuring and running linters.",