chore: drop redundant apm-marketplace-check and Copilot marketplace mirror (findings 1, 2)

apm-marketplace-check checked network reachability of remote marketplace
refs, but finding 35 already removed the only remote package entry, so
every marketplace.packages[] source is local and the hook is pure
overlap with apm-pack-check-clean. apm-audit-ci was re-examined and kept
as-is -- its pre-commit-config.yaml comment already carries a dated,
verified justification the audit had missed.

check-marketplace-mirror-sync guarded .github/plugin/marketplace.json
against drift from .claude-plugin/marketplace.json. Verified against
current GitHub Copilot CLI docs: Copilot's marketplace discovery already
falls back through .github/plugin/marketplace.json to
.claude-plugin/marketplace.json, which this repo generates anyway -- the
dedicated mirror bought a discovery-order preference, not a capability.
Deleted the mirror file, its sync script, its test, and the hook.

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) — 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.
+- **`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 manifest this repo generates or ships. Copilot CLI checks for a marketplace manifest at several conventional paths, falling back through `.github/plugin/marketplace.json` to `.claude-plugin/marketplace.json` — since this repo already generates the latter, no dedicated Copilot-path mirror is maintained.
 - **`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/gitea/references/` and `plugins/bin/evals/` — lives at the plugin **root** and is untouched by either compiler.