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

apm.yml

@@ -64,13 +64,11 @@ marketplace:
 
   # Output targets (map form). Each output writes to its profile default
   # path; add 'path:' under a key to override.
-  # 'codex' requires every package below to declare 'category:' (satisfied).
   outputs:
     claude: {}
-    codex: {}
 
-  # CI tip: build one or all formats with a machine-readable manifest:
-  #   apm pack --marketplace=claude,codex --json | jq -r '.marketplace.outputs[].path'
+  # CI tip: build a machine-readable manifest:
+  #   apm pack --marketplace=claude --json | jq -r '.marketplace.outputs[].path'
 
   versioning:
     strategy: per_package
@@ -106,12 +104,6 @@ marketplace:
       version: 1.1.2
       category: Productivity
 
-    - name: mattpocock-skills
-      description: Skills for Real Engineers — planning, TDD, architecture, and debugging workflows from Matt Pocock's .claude directory.
-      source: mattpocock/skills
-      version: "1.2.3"
-      category: Productivity
-
     - name: lint
       description: Skills and agents for configuring and running linters.
       source: ./plugins/lint