chore: remove stale check-manifests references left by e647f14

e647f14 deleted scripts/check-manifests.sh but missed three live files
that still named it: the apm-marketplace-check hook description in
.pre-commit-config.yaml, and comments in sync-plugin-content.sh and
lib/marketplace-plugins.sh explaining design decisions by pointing at
a script that no longer exists.

Also deletes list_marketplace_remote_plugin_names from
marketplace-plugins.sh — its only caller was check-manifests.sh, so
it's been dead code since that commit.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YR2CjVumUbEGWcMikcoXBD

scripts/sync-plugin-content.sh

@@ -192,8 +192,8 @@ fi
 if [[ "$ALL" -eq 1 ]]; then
   # Derives the plugin list from marketplace.json via the shared
   # list_marketplace_local_plugins helper (scripts/lib/marketplace-plugins.sh),
-  # the same one scripts/check-manifests.sh uses, instead of hand-maintaining a
-  # duplicate walk at every call site (see .pre-commit-config.yaml's
+  # a single implementation of the walk instead of hand-maintaining a
+  # duplicate at every call site (see .pre-commit-config.yaml's
   # check-plugin-content-sync).
   #
   # Hard error rather than a `|| pwd` fallback, on scripts/sync-marketplace-mirror.sh's