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

tests/test-sync-plugin-content.sh

@@ -1012,9 +1012,8 @@ make_repo_fixture() {
   printf '%s' "$marketplace_json" > "$dir/.claude-plugin/marketplace.json"
   echo "$dir"
 }
-# `env -u GIT_DIR -u GIT_WORK_TREE` for the same reason
-# tests/test-sync-marketplace-mirror.sh does it: run-tests.sh runs as a pre-push
-# hook, and git hooks export both variables, which would re-target the script's
+# `env -u GIT_DIR -u GIT_WORK_TREE`: run-tests.sh runs as a pre-push hook, and git
+# hooks export both variables, which would re-target the script's
 # `git rev-parse --show-toplevel` at the LIVE repo from any cwd.
 run_all() {
   local dir="$1"
@@ -1066,8 +1065,7 @@ check_all_fails_with "an entry whose source is an array" \
 
 # --- 28b. --all outside a git worktree refuses instead of guessing $PWD ---
 # --all's entire work list hangs off REPO_ROOT, so a `|| pwd` fallback lets it derive
-# that list from a marketplace.json belonging to some other tree. Same reasoning
-# scripts/sync-marketplace-mirror.sh dropped its own fallback on. Run from a directory
+# that list from a marketplace.json belonging to some other tree. Run from a directory
 # with no marketplace.json the old form happened to hit the "--all requires ..." error,
 # but only by accident — the dangerous case is a $PWD that HAS one.
 echo ""