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/0015-apm-replaces-plugin-marketplace-authoring.md

@@ -169,7 +169,10 @@ correction) sorted what they document into three buckets:
   apm has no version-bump automation (established under "Versioning" in issue #90's plan), so an
   ageing pin is the accepted cost of a push gate that only fires on this repo's own changes.
   Note the pin does not make the entry offline-resolvable: an exact version still requires a
-  `git ls-remote`, which is why two pre-push hooks need the network (see `AGENTS.md`).
+  `git ls-remote`, which is why two pre-push hooks needed the network (see `AGENTS.md`).
+  **Superseded 2026-09-13:** the `mattpocock-skills` entry has been removed from root `apm.yml`
+  entirely, along with the `codex` marketplace output profile. No pre-push hook needs the network
+  any longer.
 - **Caveat on "Status: executed" above:** issue #90's own execution comment flagged, before merge,
   that Claude Code's ability to actually load content out of `.apm/` was unverified — that caveat
   turned out to be a real defect, not a formality: the native installer has zero awareness of