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/spec/architecture.md

@@ -44,7 +44,7 @@ These are routing boundaries, not inventories — they answer "where does a new
 
 Two compilers produce the plugin roots you see in the tree:
 
-- **`apm pack` compiles the manifests** (ADR-0015). Per plugin: `.claude-plugin/plugin.json` and `.github/plugin/plugin.json`, both generated from `plugins/<name>/apm.yml`. Repo-wide, from the root `apm.yml`'s `marketplace:` block: `.claude-plugin/marketplace.json` (apm's `claude` output profile) and `.agents/plugins/marketplace.json` (its `codex` profile, a differently-shaped file). Those two are the only marketplace outputs apm has profiles for — the third root manifest, `.github/plugin/marketplace.json` (Copilot CLI's legacy path), is a byte-identical mirror of the Claude one maintained by `scripts/sync-marketplace-mirror.sh` and gated by the `check-marketplace-mirror-sync` pre-push hook.
+- **`apm pack` compiles the manifests** (ADR-0015). Per plugin: `.claude-plugin/plugin.json` and `.github/plugin/plugin.json`, both generated from `plugins/<name>/apm.yml`. Repo-wide, from the root `apm.yml`'s `marketplace:` block: `.claude-plugin/marketplace.json` (apm's `claude` output profile) — the only marketplace output this repo declares. A second root manifest, `.github/plugin/marketplace.json` (Copilot CLI's legacy path), is a byte-identical mirror of the Claude one maintained by `scripts/sync-marketplace-mirror.sh` and gated by the `check-marketplace-mirror-sync` pre-push hook.
 - **`scripts/sync-plugin-content.sh` compiles the content mirror** (ADR-0017). It wraps `apm pack --format plugin` and copies the resulting bundle's flat `agents/`, `skills/`, `commands/`, `instructions/`, `extensions/`, and merged `hooks/hooks.json` back to the plugin root. Claude Code's installer convention-scans those flat paths and has no `.apm/` awareness whatsoever, so the mirror exists solely to satisfy the host's discovery contract.
 
 `.apm/` is the sole hand-edited authoring source for plugin content. An edit made in the flat mirror is discarded by the next sync and is reported as drift by the `check-plugin-content-sync` pre-push hook. Hand-authored material that is not an `.apm/` primitive — `README.md`, `docs/`, `bin/`, `sources.md`, `.mcp.json`, and per-plugin extras such as `plugins/git/config.example.json`, `plugins/gitea/references/` and `plugins/bin/evals/` — lives at the plugin **root** and is untouched by either compiler.